Metronidazole (oral)

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# Black Box Warning

# Overview
Metronidazole (oral) is a anti-infective agent, antiparasitic and antiprotozoal agent that is FDA approved for the treatment of symptomatic and asymptomatic trichomoniasis, amebiasis, anaerobic bacterial infections. There is a Black Box Warning for this drug as shown here.  Common adverse reactions include abdominal discomfort, abnormal taste in mouth, diarrhea, nausea, Jarisch Herxheimer reaction, dizziness, headache, candida infection of genital region, vaginal discharge, vaginal irritation, vaginitis.

# Adult Indications and Dosage

## FDA-Labeled Indications and Dosage (Adult)
- Symptomatic Trichomoniasis.Metronidazole Tablets USP are indicated for the treatment of T. vaginalis in females and males when the presence of the trichomonad has been confirmed by appropriate laboratory procedures (wet smears and/or cultures).
- Asymptomatic Trichomoniasis.Metronidazole Tablets USP are indicated in the treatment of asymptomatic T. vaginalis infection in females when the organism is associated with endocervicitis, cervicitis, or cervical erosion. Since there is evidence that presence of the trichomonad can interfere with accurate assessment of abnormal cytological smears, additional smears should be performed after eradication of the parasite.
- Treatment of Asymptomatic Sexual Partners.T. vaginalis infection is a venereal disease. Therefore, asymptomatic sexual partners of treated patients should be treated simultaneously if the organism has been found to be present, in order to prevent reinfection of the partner. The decision as to whether to treat an asymptomatic male partner who has a negative culture or one for whom no culture has been attempted is an individual one. In making this decision, it should be noted that there is evidence that a woman may become reinfected if her sexual partner is not treated. Also, since there can be considerable difficulty in isolating the organism from the asymptomatic male carrier, negative smears and cultures cannot be relied upon in this regard. In any event, the sexual partner should be treated with Metronidazole Tablets in cases of reinfection.
- Dosing Information
- Trichomoniasis:
- In the Female:
- One-day treatment -two grams of metronidazole tablets, given either as a single dose or in two divided doses of one gram each given in the same day.
- Seven-day course of treatment -250 mg three times daily for seven consecutive days. There is some indication from controlled comparative studies that cure rates as determined by vaginal smears and signs and symptoms, may be higher after a seven-day course of treatment than after a one-day treatment regimen.
- The dosage regimen should be individualized. Single-dose treatment can assure compliance, especially if administered under supervision, in those patients who cannot be relied on to continue the seven-day regimen. A seven-day course of treatment may minimize reinfection by protecting the patient long enough for the sexual contacts to obtain appropriate treatment. Further, some patients may tolerate one treatment regimen better than the other.
- Pregnant patients should not be treated during the first trimester (see CONTRAINDICATIONS). In pregnant patients in whom alternative treatment has been inadequate, the one-day course of therapy should not be used, as it results in higher serum levels which can reach the fetal circulation (see PRECAUTIONS, Pregnancy).
- When repeat courses of the drug are required, it is recommended that an interval of four to six weeks elapse between courses and that the presence of the trichomonad be reconfirmed by appropriate laboratory measures. Total and differential leukocyte counts should be made before and after re-treatment.
- In the Male: Treatment should be individualized as for the female.
- Metronidazole tablets USP are indicated in the treatment of acute intestinal amebiasis (amebic dysentery) and amebic liver abscess.
- In amebic liver abscess, metronidazole tablet therapy does not obviate the need for aspiration or drainage of pus.
- Dosing Information
- Adults:
- For acute intestinal amebiasis (acute amebic dysentery): 750 mg orally three times daily for 5 to 10 days.
- For amebic liver abscess: 500 mg or 750 mg orally three times daily for 5 to 10 days.
- Pediatric patients:
- 35 to 50 mg/kg/24 hours, divided into three doses, orally for 10 days.

### Anaerobic Bacterial Infections
- Metronidazole tablets USP are indicated in the treatment of serious infections caused by susceptible anaerobic bacteria. Indicated surgical procedures should be performed in conjunction with Metronidazole Tablets therapy. In a mixed aerobic and anaerobic infection, antimicrobials appropriate for the treatment of the aerobic infection should be used in addition to Metronidazole Tablets.
- Dosing Information
- In the treatment of most serious anaerobic infections, intravenous metronidazole is usually administered initially.
- The usual adult oral dosage is 7.5 mg/kg every six hours (approx. 500 mg for a 70-kg adult). A maximum of 4 g should not be exceeded during a 24-hour period.
- The usual duration of therapy is 7 to 10 days; however, infections of the bone and joint, lower respiratory tract, and endocardium may require longer treatment.

### INTRA-ABDOMINAL INFECTIONS
- INTRA-ABDOMINAL INFECTIONS, including peritonitis, intra-abdominal abscess, and liver abscess, caused by Bacteroides species including the B. fragilis group (B. fragilis, B. distasonis, B. ovatus, B. thetaiotaomicron, B. vulgatus), Clostridium species, Eubacterium species, Peptococcus species, and Peptostreptococcus species.

### SKIN AND SKIN STRUCTURE INFECTIONS
- SKIN AND SKIN STRUCTURE INFECTIONS caused by Bacteroides species including the B. fragilis group, Clostridium species, Peptococcus species, Peptostreptococcus species, and Fusobacterium species.

### GYNECOLOGIC INFECTIONS
- GYNECOLOGIC INFECTIONS, including endometritis, endomyometritis, tubo-ovarian abscess, and postsurgical vaginal cuff infection, caused by Bacteroides species including the B. fragilis group, Clostridium species, Peptococcus species,Peptostreptococcus species, and Fusobacterium species.

### BACTERIAL SEPTICEMIA
- BACTERIAL SEPTICEMIA caused by Bacteroides species including the B. fragilis group, and Clostridium species.

### BONE AND JOINT INFECTIONS
- BONE AND JOINT INFECTIONS, as adjunctive therapy, caused by Bacteroides species including the B. fragilis group.

### CENTRAL NERVOUS SYSTEM (CNS) INFECTIONS
- CENTRAL NERVOUS SYSTEM (CNS) INFECTIONS, including meningitis and brain abscess, caused by Bacteroides species including the B. fragilis group.

### LOWER RESPIRATORY TRACT INFECTIONS
- LOWER RESPIRATORY TRACT INFECTIONS, including pneumonia, empyema, and lung abscess, caused by Bacteroides species including the B. fragilis group.

### ENDOCARDITIS
- ENDOCARDITIS caused by Bacteroides species including the B. fragilis group.
- To reduce the development of drug-resistant bacteria and maintain the effectiveness of metronidazole tablets USP and other antibacterial drugs, metronidazole tablets USP should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.

## Off-Label Use and Dosage (Adult)

### Guideline-Supported Use
- There is limited information regarding Off-Label Guideline-Supported Use of Metronidazole (oral) in adult patients.

### NonGuideline-Supported Use

### Clostridium difficile diarrhea, including Pseudomembranous Colitis
- Dosing Information
- Intravenous route:
- Guideline Dosing:
- Based on expert opinion, the recommended treatment for severe, complicated Clostridium difficile infection is metronidazole 500 mg IV every 8 hours in combination with vancomycin administered orally by mouth or by nasogastric tube, according to the Society for Healthcare Epidemiology of America (SHEA) and the Infectious Diseases Society of America (IDSA) clinical practice guidelines.
- Intravenous metronidazole (500 mg 3 times daily for 4 to 7 days) has been effective for the treatment of pseudomembranous colitis in patients who are unable to tolerate oral therapy. However, parenteral metronidazole is less effective than oral metronidazole in the prevention and treatment of antibiotic-associated pseudomembranous colitis.
- Oral route:
- Guideline Dosing:
- Metronidazole 500 mg orally 3 times per day for 10 to 14 days is the recommended treatment for the initial episode and first recurrence of mild-to-moderate Clostridium difficile infection in adults, according to the Society for Healthcare Epidemiology of America (SHEA) and the Infectious Diseases Society of America (IDSA) guidelines.

### Giardiasis
- Dosing Information
- Metronidazole is considered by many to be the drug of choice in giardiasis due to its similar efficacy and lower degree of toxicity as compared to quinacrine. The usual dose is 250 mg orally 3 times a day for 5 days.

### Helicobacter pylori gastrointestinal tract infection
- Dosing Information
- Quadruple Therapy:
- The recommended dose is metronidazole 250 mg orally 4 times a day, bismuth subsalicylate 525 mg orally 4 times a day, tetracycline 500 mg orally 4 times a day, and ranitidine 150 mg orally twice a day or standard dose of a proton pump inhibitor for 10 to 14 days. Standard dose proton pump inhibitors include lansoprazole 30 mg twice daily, omeprazole 20 mg twice daily, pantoprazole 40 mg twice daily, rabeprazole 20 mg twice daily, or esomeprazole 40 mg once daily.
- Triple Therapy:
- The recommended dose is metronidazole 500 mg orally twice a day in combination with clarithromycin 500 mg orally twice a day and standard-dose proton pump inhibitors for 10 to 14 days. Standard dose proton pump inhibitors include lansoprazole 30 mg twice daily, omeprazole 20 mg twice daily, pantoprazole 40 mg twice daily, rabeprazole 20 mg twice daily, or esomeprazole 40 mg once daily.

### Infection by Fasciola

### Sexually transmitted infectious disease; Prophylaxis - Victim of sexual aggression
- Dosing Information
- The recommended regimen for preventing sexually transmitted diseases following sexual assault is a single dose plus metronidazole 2 g orally plus a single dose of ceftriaxone 250 mg IM or cefixime 400 mg orally plus either azithromycin 1 g orally in a single dose or doxycycline 100 mg orally twice daily for 7 days.

# Pediatric Indications and Dosage

## FDA-Labeled Indications and Dosage (Pediatric)
- Dosing Information
- For treatment of amebic dysentery in children, the recommended dose of metronidazole is 35 to 50 mg/kg/day orally in 3 divided doses for 10 days.
- Dosing Information
- For treatment of amebic liver abscess in children, the recommended dose of metronidazole is 35 to 50 mg/kg/day orally in 3 divided doses (up to 750 mg/dose) for 10 days in conjunction with appropriate aspiration or drainage of pus.
There is limited information regarding FDA-Labeled Use of Metronidazole (oral) in pediatric patients.

## Off-Label Use and Dosage (Pediatric)

### Guideline-Supported Use
- There is limited information regarding Off-Label Guideline-Supported Use of Metronidazole (oral) in pediatric patients.

### NonGuideline-Supported Use

### Giardiasis
- Dosing Information
- The recommended oral dose of metronidazole in the treatment of giardiasis (Giardia lamblia) in children is 15 mg/kg/24 hours, up to 750 mg/24 hours, in 3 divided doses for 7 to 10 days.

### Helicobacter pylori gastrointestinal tract infection

# Contraindications
Hypersensitivity:
- Metronidazole tablets are contraindicated in patients with a prior history of hypersensitivity to metronidazole or other nitroimidazole derivatives.
- In patients with trichomoniasis, metronidazole tablets are contraindicated during the first trimester of pregnancy. (see PRECAUTIONS.)
Psychotic Reaction with Disulfiram:
- Use of oral metronidazole is associated with psychotic reactions in alcoholic patients who were using disulfiram concurrently. Do not administer metronidazole to patients who have taken disulfiram within the last two weeks.
Interaction with Alcohol:
- Use of oral metronidazole is associated with a disulfiram-like reaction to alcohol, including abdominal cramps, nausea, vomiting, headaches, and flushing. Discontinue consumption of alcohol or products containing propylene glycol during and for at least three days after therapy with metronidazole (see PRECAUTIONS, Drug Interactions).

# Warnings
Central and Peripheral Nervous System Effects
- Encephalopathy and peripheral neuropathy: Cases of encephalopathy and peripheral neuropathy (including optic neuropathy) have been reported with metronidazole.
- Encephalopathy has been reported in association with cerebellar toxicity characterized by ataxia, dizziness, and dysarthria. CNS lesions seen on MRI have been described in reports of encephalopathy. CNS symptoms are generally reversible within days to weeks upon discontinuation of metronidazole. CNS lesions seen on MRI have also been described as reversible.
- Peripheral neuropathy, mainly of sensory type has been reported and is characterized by numbness or paresthesia of an extremity.
- Convulsive seizures have been reported in patients treated with metronidazole.
- Aseptic meningitis: Cases of aseptic meningitis have been reported with metronidazole. Symptoms can occur within hours of dose administration and generally resolve after metronidazole therapy is discontinued.
- The appearance of abnormal neurologic signs and symptoms demands the prompt evaluation of the benefit/risk ratio of the continuation of therapy (see ADVERSE REACTIONS).
PRECAUTIONS
General
Hepatic Impairment
- Patients with hepatic impairment metabolize metronidazole slowly, with resultant accumulation of metronidazole in the plasma. For patients with severe hepatic impairment (Child-Pugh C), a reduced dose of metronidazole is recommended. For patients with mild to moderate hepatic impairment, no dosage adjustment is needed but these patients should be monitored for metronidazole associated adverse events.
Renal Impairment
- Patients with end-stage renal disease may excrete metronidazole and metabolites slowly in the urine, resulting in significant accumulation of metronidazole metabolites. Monitoring for metronidazole associated adverse events is recommended.
Fungal Superinfections
- Known or previously unrecognized candidiasis may present more prominent symptoms during therapy with metronidazole and requires treatment with a candidacidal agent.
Use in Patients with Blood Dyscrasias
- Metronidazole is a nitroimidazole and should be used with caution in patients with evidence of or history of blood dyscrasia. A mild leukopenia has been observed during its administration; however, no persistent hematologic abnormalities attributable to metronidazole have been observed in clinical studies. Total and differential leukocyte counts are recommended before and after therapy.
Drug-Resistant Bacteria and Parasites
- Prescribing metronidazole tablets in the absence of a proven or strongly suspected bacterial or parasitic infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria and parasites.
Information for patients
Interaction with Alcohol
- Discontinue consumption of alcoholic beverages or products containing propylene glycol while taking metronidazole tablets and for at least three days afterward because abdominal cramps, nausea, vomiting, headaches, and flushing may occur.
Treatment of Bacterial and Parasitic Infections
- Patients should be counseled that metronidazole tablets should only be used to treat bacterial and parasitic infections. Metronidazole tablets do not treat viral infections (e.g., the common cold). When metronidazole tablets is prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed. Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by Metronidazole Tablets in the future.
Drug/Laboratory test interactions
- Metronidazole may interfere with certain types of determinations of serum chemistry values, such as aspartate aminotransferase (AST, SGOT), alanine aminotransferase (ALT, SGPT), lactate dehydrogenase (LDH), triglycerides, and glucose hexokinase. Values of zero may be observed. All of the assays in which interference has been reported involve enzymatic coupling of the assay to oxidation-reduction of nicotinamide adenine dinucleotide (NAD+   NADH). Interference is due to the similarity in absorbance peaks of NADH (340 nm) and metronidazole (322 nm) at pH 7.

# Adverse Reactions

## Clinical Trials Experience
- The following reactions have been reported during treatment with metronidazole:
- Central Nervous System: The most serious adverse reactions reported in patients treated with metronidazole have been convulsive seizures, encephalopathy, aseptic meningitis, optic and peripheral neuropathy, the latter characterized mainly by numbness or paresthesia of an extremity. Since persistent peripheral neuropathy has been reported in some patients receiving prolonged administration of metronidazole, patients should be specifically warned about these reactions and should be told to stop the drug and report immediately to their physicians if any neurologic symptoms occur. In addition, patients have reported headache, syncope, dizziness, vertigo, incoordination, ataxia, confusion, dysarthria, irritability, depression, weakness, and insomnia (see WARNINGS).
- Gastrointestinal:The most common adverse reactions reported have been referable to the gastrointestinal tract, particularly nausea, sometimes accompanied by headache, anorexia, and occasionally vomiting; diarrhea; epigastric distress; and abdominal cramping and constipation.
- Mouth: A sharp, unpleasant metallic taste is not unusual. Furry tongue, glossitis, and stomatitis have occurred; these may be associated with a sudden overgrowth of Candida which may occur during therapy.
- Dermatologic: Erythematous rash and pruritus.
- Hematopoietic: Reversible neutropenia (leukopenia); rarely, reversible thrombocytopenia.
- Cardiovascular: Flattening of the T-wave may be seen in electrocardiographic tracings.
- Hypersensitivity: Urticaria, erythematous rash, Stevens-Johnson Syndrome, toxic epidermal necrolysis, flushing, nasal congestion, dryness of the mouth (or vagina or vulva), and fever.
- Renal: Dysuria, cystitis, polyuria, incontinence, and a sense of pelvic pressure. Instances of darkened urine have been reported by approximately one patient in 100,000. Although the pigment which is probably responsible for this phenomenon has not been positively identified, it is almost certainly a metabolite of metronidazole and seems to have no clinical significance.
- Other: Proliferation of Candida in the vagina, dyspareunia, decrease of libido, proctitis, and fleeting joint pains sometimes resembling serum sickness. Rare cases of pancreatitis, which generally abated on withdrawal of the drug, have been reported.
- Patients with Crohns disease are known to have an increased incidence of gastrointestinal and certain extraintestinal cancers. There have been some reports in the medical literature of breast and colon cancer in Crohns disease patients who have been treated with metronidazole at high doses for extended periods of time. A cause and effect relationship has not been established. Crohns disease is not an approved indication for Metronidazole Tablets.

## Postmarketing Experience
There is limited information regarding Metronidazole (oral) Postmarketing Experience in the drug label.

# Drug Interactions
Disulfiram
- Psychotic reactions have been reported in alcoholic patients who are using metronidazole and disulfiram concurrently. Metronidazole should not be given to patients who have taken disulfiram within the last two weeks (see CONTRAINDICATIONS).
Alcoholic Beverages
- Abdominal cramps, nausea, vomiting, headaches, and flushing may occur if alcoholic beverages or products containing propylene glycol are consumed during or following metronidazole therapy (see CONTRAINDICATIONS).
Warfarin and other Oral Anticoagulants
- Metronidazole has been reported to potentiate the anticoagulant effect of warfarin and other oral coumarin anticoagulants, resulting in a prolongation of prothrombin time. When metronidazole tablets are prescribed for patients on this type of anticoagulant therapy, prothrombin time and INR should be carefully monitored.
Lithium
- In patients stabilized on relatively high doses of lithium, short-term metronidazole therapy has been associated with elevation of serum lithium and, in a few cases, signs of lithium toxicity. Serum lithium and serum creatinine levels should be obtained several days after beginning metronidazole to detect any increase that may precede clinical symptoms of lithium intoxication.
Busulfan
- Metronidazole has been reported to increase plasma concentrations of busulfan, which can result in an increased risk for serious busulfan toxicity. Metronidazole should not be administered concomitantly with busulfan unless the benefit outweighs the risk. If no therapeutic alternatives to metronidazole are available, and concomitant administration with busulfan is medically needed, frequent monitoring of busulfan plasma concentration should be performed and the busulfan dose should be adjusted accordingly.
Drugs that Inhibit CYP450 Enzymes
- The simultaneous administration of drugs that decrease microsomal liver enzyme activity, such as cimetidine, may prolong the half-life and decrease plasma clearance of metronidazole.
Drugs that Induce CYP450 Enzymes
- The simultaneous administration of drugs that induce microsomal liver enzymes, such as phenytoin or phenobarbital, may accelerate the elimination of metronidazole, resulting in reduced plasma levels; impaired clearance of phenytoin has also been reported.

# Use in Specific Populations

### Pregnancy
Pregnancy Category (FDA): B
Teratogenic Effects : Pregnancy Category B
- There are no adequate and well controlled studies of metronidazole in pregnant women. There are published data from case-control studies, cohort studies, and 2 meta-analyses that include more than 5000 pregnant women who used metronidazole during pregnancy. Many studies included first trimester exposures. One study showed an increased risk of cleft lip, with or without cleft palate, in infants exposed to metronidazole in-utero; however, these findings were not confirmed. In addition, more than ten randomized placebo-controlled clinical trials enrolled more than 5000 pregnant women to assess the use of antibiotic treatment (including metronidazole) for bacterial vaginosis on the incidence of preterm delivery. Most studies did not show an increased risk for congenital anomalies or other adverse fetal outcomes following metronidazole exposure during pregnancy. Three studies conducted to assess the risk of infant cancer following metronidazole exposure during pregnancy did not show an increased risk; however, the ability of these studies to detect such a signal was limited.
- Metronidazole crosses the placental barrier and its effects on the human fetal organogenesis are not known. Reproduction studies have been performed in rats, rabbits, and mice at doses similar to the maximum recommended human dose based on body surface area comparisons. There was no evidence of harm to the fetus due to metronidazole.
Pregnancy Category (AUS):
- Australian Drug Evaluation Committee (ADEC) Pregnancy Category
- There is no Australian Drug Evaluation Committee (ADEC) guidance on usage of Metronidazole (oral) in women who are pregnant.

### Labor and Delivery
- There is no FDA guidance on use of Metronidazole (oral) during labor and delivery.

### Nursing Mothers
- Metronidazole is present in human milk at concentrations similar to maternal serum levels, and infant serum levels can be close to or comparable to infant therapeutic levels. Because of the potential for tumorigenicity shown for metronidazole in mouse and rat studies, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother. Alternatively, a nursing mother may choose to pump and discard human milk for the duration of metronidazole therapy, and for 24 hours after therapy ends and feed her infant stored human milk or formula.

### Pediatric Use
- Safety and effectiveness in pediatric patients have not been established, except for the treatment of amebiasis.

### Geriatic Use
- In elderly geriatric patients, monitoring for metronidazole associated adverse events is recommended (see CLINICAL PHARMACOLOGY, PRECAUTIONS). Decreased liver function in geriatric patients can result in increased concentrations of metronidazole that may necessitate adjustment of metronidazole dosage (see DOSAGE AND ADMINISTRATION).

### Gender
- There is no FDA guidance on the use of Metronidazole (oral) with respect to specific gender populations.

### Race
- There is no FDA guidance on the use of Metronidazole (oral) with respect to specific racial populations.

### Renal Impairment
- There is no FDA guidance on the use of Metronidazole (oral) in patients with renal impairment.

### Hepatic Impairment
- There is no FDA guidance on the use of Metronidazole (oral) in patients with hepatic impairment.

### Females of Reproductive Potential and Males
- There is no FDA guidance on the use of Metronidazole (oral) in women of reproductive potentials and males.

### Immunocompromised Patients
- There is no FDA guidance one the use of Metronidazole (oral) in patients who are immunocompromised.

# Administration and Monitoring

### Administration
Trichomoniasis:
- In the Female:
- One-day treatment -two grams of metronidazole tablets, given either as a single dose or in two divided doses of one gram each given in the same day.
- Seven-day course of treatment -250 mg three times daily for seven consecutive days. There is some indication from controlled comparative studies that cure rates as determined by vaginal smears and signs and symptoms, may be higher after a seven-day course of treatment than after a one-day treatment regimen.
- The dosage regimen should be individualized. Single-dose treatment can assure compliance, especially if administered under supervision, in those patients who cannot be relied on to continue the seven-day regimen. A seven-day course of treatment may minimize reinfection by protecting the patient long enough for the sexual contacts to obtain appropriate treatment. Further, some patients may tolerate one treatment regimen better than the other.
- Pregnant patients should not be treated during the first trimester (see CONTRAINDICATIONS). In pregnant patients in whom alternative treatment has been inadequate, the one-day course of therapy should not be used, as it results in higher serum levels which can reach the fetal circulation (see PRECAUTIONS, Pregnancy).
- When repeat courses of the drug are required, it is recommended that an interval of four to six weeks elapse between courses and that the presence of the trichomonad be reconfirmed by appropriate laboratory measures. Total and differential leukocyte counts should be made before and after re-treatment.
- In the Male: Treatment should be individualized as for the female.
Amebiasis:
Adults:
- For acute intestinal amebiasis (acute amebic dysentery): 750 mg orally three times daily for 5 to 10 days.
- For amebic liver abscess: 500 mg or 750 mg orally three times daily for 5 to 10 days.
Pediatric patients
- 35 to 50 mg/kg/24 hours, divided into three doses, orally for 10 days.
Anaerobic Bacterial Infections
- In the treatment of most serious anaerobic infections, intravenous metronidazole is usually administered initially.
- The usual adult oral dosage is 7.5 mg/kg every six hours (approx. 500 mg for a 70-kg adult). A maximum of 4 g should not be exceeded during a 24-hour period.
- The usual duration of therapy is 7 to 10 days; however, infections of the bone and joint, lower respiratory tract, and endocardium may require longer treatment.
Dosage Adjustments
Patients with Severe Hepatic Impairment
- For patients with severe hepatic impairment (Child-Pugh C), the dose of metronidazole tablets should be reduced by 50% (see CLINICAL PHARMACOLOGY and PRECAUTIONS).
Patients Undergoing Hemodialysis:
- Hemodialysis removes significant amounts of metronidazole and its metabolites from systemic circulation. The clearance of metronidazole will depend on the type of dialysis membrane used, the duration of the dialysis session, and other factors. If the administration of metronidazole cannot be separated from the hemodialysis session, supplementation of metronidazole dosage following the hemodialysis session should be considered, depending on the patients clinical situation (see PHARMACOLOGY).

### Monitoring
- On account of the potential accumulation of metronidazole metabolites in ESRD patients, monitoring for metronidazole associated adverse events is recommended.
- Patients with mild to moderate hepatic impairment should be monitored for metronidazole associated adverse events.
- In geriatric patients, monitoring for metronidazole associated adverse events is recommended.
- For patients with mild to moderate hepatic impairment, no dosage adjustment is needed but these patients should be monitored for metronidazole associated adverse events.
- Patients with end-stage renal disease may excrete metronidazole and metabolites slowly in the urine, resulting in significant accumulation of metronidazole metabolites. Monitoring for metronidazole associated adverse events is recommended.
- When metronidazole tablets are prescribed for patients on this type of anticoagulant therapy, prothrombin time and INR should be carefully monitored.
- If no therapeutic alternatives to metronidazole are available, and concomitant administration with busulfan is medically needed, frequent monitoring of busulfan plasma concentration should be performed and the busulfan dose should be adjusted accordingly.
- In elderly geriatric patients, monitoring for metronidazole associated adverse events is recommended (see PHARMACOLOGY, PRECAUTIONS). Decreased liver function in geriatric patients can result in increased concentrations of metronidazole that may necessitate adjustment of metronidazole dosage (see DOSAGE AND ADMINISTRATION).

# IV Compatibility
- There is limited information regarding IV Compatibility of Metronidazole (oral) in the drug label.

# Overdosage
- Single oral doses of metronidazole, up to 15 g, have been reported in suicide attempts and accidental overdoses. Symptoms reported include nausea, vomiting, and ataxia.
- Oral metronidazole has been studied as a radiation sensitizer in the treatment of malignant tumors. Neurotoxic effects, including seizures and peripheral neuropathy, have been reported after 5 to 7 days of doses of 6 to 10.4 g every other day.
- Treatment of Overdosage: There is no specific antidote for metronidazole Tablets overdose; therefore, management of the patient should consist of symptomatic and supportive therapy.

# Pharmacology

## Mechanism of Action
There is limited information regarding Metronidazole (oral) Mechanism of Action in the drug label.

## Structure
- Metronidazole Tablets USP, 250 mg or 500 mg is an oral formulation of the synthetic nitroimidazole antimicrobial, 2-methyl-5-nitro-1H-imidazole-1-ethanol, which has the following structural formula:
- Metronidazole 250 mg and 500 mg tablets USP, for oral administration, contain the inactive ingredients:. colloidal silicon dioxide, hydroxypropyl cellulose, lactose (anhydrous), microcrystalline cellulose, sodium starch glycolate, and stearic acid.

## Pharmacodynamics
- There is limited information regarding Pharmacodynamics of Metronidazole (oral) in the drug label.

## Pharmacokinetics
Absorption
- Disposition of metronidazole in the body is similar for both oral and intravenous dosage forms. Following oral administration, metronidazole is well absorbed, with peak plasma concentrations occurring between one and two hours after administration.
- Plasma concentrations of metronidazole are proportional to the administered dose. Oral administration of 250 mg, 500 mg, or 2,000 mg produced peak plasma concentrations of 6 mcg/mL, 12 mcg/mL, and 40 mcg/mL, respectively. Studies reveal no significant bioavailability differences between males and females; however, because of weight differences, the resulting plasma levels in males are generally lower.
Distribution
- Metronidazole is the major component appearing in the plasma, with lesser quantities of metabolites also being present. Less than 20% of the circulating metronidazole is bound to plasma proteins. Metronidazole appears in cerebrospinal fluid, saliva, and breast milk in concentrations similar to those found in plasma. Bactericidal concentrations of metronidazole have also been detected in pus from hepatic abscesses.
Metabolism/Excretion
- The major route of elimination of metronidazole and its metabolites is via the urine (60% to 80% of the dose), with fecal excretion accounting for 6% to 15% of the dose. The metabolites that appear in the urine result primarily from side-chain oxidation  and glucuronide conjugation, with unchanged metronidazole accounting for approximately 20% of the total.  Both the parent compound and the hydroxyl metabolite possess in vitro antimicrobial activity.
- Renal clearance of metronidazole is approximately 10 mL/min/ 1.73 m2. The average elimination half-life of metronidazole in healthy subjects is eight hours.
Renal Impairment
- Decreased renal function does not alter the single-dose pharmacokinetics of metronidazole.
- Subjects with end-stage renal disease (ESRD; CLCR= 8.19.1 mL/min) and who received a single intravenous infusion of metronidazole 500 mg had no significant change in metronidazole pharmacokinetics but had 2-fold higher Cmax of hydroxy-metronidazole and 5-fold higher Cmax of metronidazole acetate, compared to healthy subjects with normal renal function (CLCR= 12616 mL/min). Thus, on account of the potential accumulation of metronidazole metabolites in ESRD patients, monitoring for metronidazole associated adverse events is recommended (see PRECAUTIONS).
Effect of Dialysis
- Following a single intravenous infusion or oral dose of metronidazole 500 mg, the clearance of metronidazole was investigated in ESRD subjects undergoing hemodialysis or continuous ambulatory peritoneal dialysis (CAPD). A hemodialysis session lasting for 4 to 8 hours removed 40% to 65% of the administered metronidazole dose, depending on the type of dialyzer membrane used and the duration of the dialysis session. If the administration of metronidazole cannot be separated from the dialysis session, supplementation of metronidazole dose following hemodialysis should be considered. A peritoneal dialysis session lasting for 7.5 hours removed approximately 10% of the administered metronidazole dose. No adjustment in metronidazole dose is needed in ESRD patients undergoing CAPD.
Hepatic Impairment
- Following a single intravenous infusion of 500 mg metronidazole, the mean AUC24 of metronidazole was higher by 114% in patients with severe (Child-Pugh C) hepatic impairment, and by 54% and 53% in patients with mild (Child-Pugh A) and moderate (Child-Pugh B) hepatic impairment, respectively, compared to healthy control subjects. There were no significant changes in the AUC24 of hydroxyl-metronidazole in these hepatically impaired patients. A reduction in metronidazole dosage by 50% is recommended in patients with severe (Child-Pugh C) hepatic impairment (see DOSAGE AND ADMINISTRATION). No dosage adjustment is needed for patients with mild to moderate hepatic impairment. Patients with mild to moderate hepatic impairment should be monitored for metronidazole associated adverse events (see PRECAUTIONS).
Geriatric Patients
- Following a single 500 mg oral or IV dose of metronidazole, subjects >70 years old with no apparent renal or hepatic dysfunction had a 40% to 80% higher mean AUC of hydroxy-metronidazole (active metabolite), with no apparent increase in the mean AUC of metronidazole (parent compound), compared to young healthy controls <40 years old. In geriatric patients, monitoring for metronidazole associated adverse events is recommended (see PRECAUTIONS).
Pediatric Patients
- In one study, newborn infants appeared to demonstrate diminished capacity to eliminate metronidazole. The elimination half-life, measured during the first 3 days of life, was inversely related to gestational age. In infants whose gestational ages were between 28 and 40 weeks, the corresponding elimination half-lives ranged from 109 to 22.5 hours.
Microbiology:
Mechanism of Action
- Metronidazole exerts antibacterial effects in an anaerobic environment by the following possible mechanism: Once metronidazole enters the organism, the drug is reduced by intracellular electron transport proteins. Because of this alteration to the metronidazole molecule, a concentration gradient is created and maintained which promotes the drugs intracellular transport. Presumably, free radicals are formed which, in turn, react with cellular components resulting in death of the bacteria.
- Metronidazole is active against most obligate anaerobes, but does not possess any clinically relevant activity against facultative anaerobes or obligate aerobes.
Activity In Vitro and In Vivo
- Metronidazole has been shown to be active against most isolates of the following bacteria both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.
Gram-positive anaerobes
- Clostridium species
- Eubacterium species
- Peptococcus species
- Peptostreptococcus species
Gram-negative anaerobes
- Bacteroides fragilis group (B. fragilis, B. distasonis, B. ovatus, B.     thetaiotaomicron, B.vulgatus)
- Fusobacterium species
Protozoal parasites
- Entamoeba histolytica
- Trichomonas vaginalis
- The following in vitro data are available, but their clinical significance is unknown:
- Metronidazole exhibits in vitro minimum inhibitory concentrations (MICs) of <8 mcg/mL or less against most ( 90%) isolates of the following bacteria; however, the safety and effectiveness of metronidazole in treating clinical infections due to these bacteria have not been established in adequate and well-controlled clinical trials.
Gram-negative anaerobes
- Bacteroides fragilis group (B. caccae, B. uniformis)
- Prevotella species (P. bivia, P. buccae, P. disiens)
Susceptibility Test Methods:
- When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drug products used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug product for treatment.
Anaerobic Techniques
- Quantitative methods are used to determine minimum inhibitory concentrations provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. For anaerobic bacteria susceptibility to metronidazole can be determined by the reference broth or agar dilution method1,2. The MIC values obtained should be interpreted according to the following criteria:
- For protozoal parasites:Standardized tests do not exist for use in clinical microbiology laboratories.
- A report of Susceptible indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the infection site necessary to inhibit growth of the pathogen. A report of Intermediate indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where a high dosage of the drug product is physiologically concentrated or in situations where a high dosage of the drug product can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.
Quality Control
- Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test.1,2 Standard metronidazole powder should provide a value within the MIC ranges noted in the following table:

## Nonclinical Toxicology
Carcinogenesis, mutagenesis, impairment of fertility:
- Tumors affecting the liver, lungs, mammary, and lymphatic tissues have been detected in several studies of metronidazole in rats and mice, but not hamsters.
- Pulmonary tumors have been observed in all six reported studies in the mouse, including one study in which the animals were dosed on an intermittent schedule (administration during every fourth week only). Malignant liver tumors were increased in male mice treated at approximately 1500 mg/m2 (similar to the maximum recommended daily dose, based on body surface area comparisons). Malignant lymphomas and pulmonary neoplasms were also increased with lifetime feeding of the drug to mice. Mammary and hepatic tumors were increased among female rats administered oral metronidazole compared to concurrent controls. Two lifetime tumorigenicity studies in hamsters have been performed and reported to be negative.
- Metronidazole has shown mutagenic activity in in vitro assay systems including the Ames test. Studies in mammals in vivo have failed to demonstrate a potential for genetic damage.
- Metronidazole failed to produce any adverse effects on fertility or testicular function in male rats at doses up at 400 mg/kg/day (similar to the maximum recommended clinical dose, based on body surface area comparisons) for 28 days. However, rats treated at the same dose for 6 weeks or longer were infertile and showed severe degeneration of the seminiferous epithelium in the testes as well as marked decreases in testicular spermatid counts and epididymal sperm counts. Fertility was restored in most rats after an eight week, drug-free recovery period.

# Clinical Studies
There is limited information regarding Clinical Studies of Metronidazole (oral) in the drug label.

# How Supplied
- Metronidazole Tablets USP, 250 mg are available as white, round, convex tablets debossed with 3969 on one side and WPI on the other side and are packaged in bottles of 100 (NDC 68001-229-00), 250 (NDC 68001-229-13), and 500 (NDC 68001-229-03). Metronidazole tablets USP, 500mg are available as white, oblong tablets, debossed with 3970 on one side and WPI on the other side and are packaged in bottles of 500 (NDC 68001-263-03). Dispense in a tight, light-resistant container as defined in the USP with a child-resistant closure (as required).

## Storage
- Store at 20-25C (68-77F) . Preserve in well closed light resistant container.

# Images

## Drug Images

## Package and Label Display Panel

# Patient Counseling Information
Information for patients
Interaction with Alcohol
- Discontinue consumption of alcoholic beverages or products containing propylene glycol while taking metronidazole tablets and for at least three days afterward because abdominal cramps, nausea, vomiting, headaches, and flushing may occur (see CONTRAINDICATIONS and PRECAUTIONS, Drug Interactions).
Treatment of Bacterial and Parasitic Infections
- Patients should be counseled that metronidazole tablets should only be used to treat bacterial and parasitic infections. Metronidazole tablets do not treat viral infections (e.g., the common cold). When metronidazole tablets is prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed. Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by Metronidazole Tablets in the future.

# Precautions with Alcohol
Disulfiram
- Psychotic reactions have been reported in alcoholic patients who are using metronidazole and disulfiram concurrently. Metronidazole should not be given to patients who have taken disulfiram within the last two weeks (see CONTRAINDICATIONS).
Alcoholic Beverages
- Abdominal cramps, nausea, vomiting, headaches, and flushing may occur if alcoholic beverages or products containing propylene glycol are consumed during or following metronidazole therapy (see CONTRAINDICATIONS).

# Brand Names
Flagyl,
Flagyl ER,
Flagyl I.V. RTU,
Metrocream,
Metrogel,
Metrogel-Vaginal,
Metrolotion,
Noritate.

# Look-Alike Drug Names
- A  B

# Drug Shortage Status

# Price