# Cyproheptadine

Cyproheptadine, sold under the brand name Periactin among others, is a first-generation antihistamine with additional anticholinergic, antiserotonergic, and local anesthetic properties.
It was patented in 1959 and came into medical use in 1961. In 2022, it was the 293rd most commonly prescribed medication in the United States, with more than 400,000 prescriptions.

## Medical uses

Cyproheptadine is used to treat allergic reactions (specifically hay fever). There is evidence supporting its use for allergies, but second generation antihistamines such as ketotifen and loratadine have shown equal results with fewer side effects.
It is also used as a preventive treatment against migraine. In a 2013 study the frequency of migraine was dramatically reduced in patients within 7 to 10 days after starting treatment. The average frequency of migraine attacks in these patients before administration was 8.7 times per month, this was decreased to 3.1 times per month at 3 months after the start of treatment. This use is on the label in the UK and some other countries.
It is also used off-label in the treatment of cyclical vomiting syndrome in infants; the only evidence for this use comes from retrospective studies.
Cyproheptadine is sometimes used off-label to improve akathisia in people on antipsychotic medications.
It is used off-label to treat various dermatological conditions, including psychogenic itch, drug-induced hyperhidrosis (excessive sweating), and prevention of blister formation for some people with epidermolysis bullosa simplex.
One of the effects of the drug is increased appetite and weight gain, which has led to its use (off-label in the USA) for this purpose in children who are wasting as well as people with cystic fibrosis.
It is also used off-label in the management of moderate to severe cases of serotonin syndrome, a complex of symptoms associated with the use of serotonergic drugs, such as selective serotonin reuptake inhibitors (and monoamine oxidase inhibitors), and in cases of high levels of serotonin in the blood resulting from a serotonin-producing carcinoid tumor.
Cyproheptadine has sedative effects and can be used to treat insomnia similarly to other centrally-acting antihistamines. The recommended dose for this use is 4 to 8 mg.

## Adverse effects

Adverse effects include:
* Sedation and sleepiness (often transient)
* Dizziness
* Disturbed coordination
* Confusion
* Restlessness
* Excitation
* Nervousness
* Tremor
* Irritability
* Insomnia
* Paresthesias
* Neuritis
* Convulsions
* Euphoria
* Hallucinations
* Hysteria
* Faintness
* Allergic manifestation of rash and edema
* Diaphoresis
* Urticaria
* Photosensitivity
* Acute labyrinthitis
* Diplopia (seeing double)
* Vertigo
* Tinnitus
* Hypotension (low blood pressure)
* Palpitation
* Extrasystoles
* Anaphylactic shock
* Hemolytic anemia
* Blood dyscrasias such as leukopenia, agranulocytosis and thrombocytopenia
* Cholestasis
* Hepatic (liver) side effects such as:
** Hepatitis
** Jaundice
** Liver failure
** Hepatic function abnormality
* Epigastric distress
* Anorexia
* Nausea
* Vomiting
* Diarrhea
* Anticholinergic side effects such as:
** Blurred vision
** Constipation
** Xerostomia (dry mouth)
** Tachycardia (high heart rate)
** Urinary retention
** Difficulty passing urine
** Nasal congestion
** Nasal or throat dryness
* Urinary frequency
* Early menses
* Thickening of bronchial secretions
* Tightness of chest and wheezing
* Fatigue
* Chills
* Headache
* Increased appetite
* Weight gain

### Overdose

Gastric decontamination measures such as activated charcoal are sometimes recommended in cases of overdose. The symptoms are usually indicative of CNS depression (or conversely CNS stimulation in some) and excess anticholinergic side effects. The LD$_{50}$ in mice is 123 mg/kg and 295 mg/kg in rats.

## Pharmacology
### Pharmacodynamics

Cyproheptadine :
| Site | K$_i$ (nM) | Action | Species |
| 5-HT$_{1A}$ | 50–59 | ↓ | Human |
| 5-HT$_{1B}$ | 1600 | | Human |
| 5-HT$_{1D}$ | 670 | | Human |
| 5-HT$_{1E}$ | 1500 | | Human |
| 5-HT$_{2A}$ | 0.46–3.0 | ↓ | Human |
| 5-HT$_{2B}$ | 1.5–2.6 | ↓ | Human |
| 5-HT$_{2C}$ | 2.2–18 | ↓ | Human |
| 5-HT$_3$ | 235 | | Human |
| 5-HT$_4$ | ND | | Human |
| 5-HT$_{5A}$ | 57 | | Human |
| 5-HT$_6$ | 96–150 | | Human |
| 5-HT$_7$ | 30–126 | | Human |
| D$_1$ | 10–117 | | Human |
| D$_2$ | 74–112 | ↓ | Human |
| D$_3$ | 8 | | Human |
| D$_4$ | 120 | | Human |
| D$_5$ | 60 | | Human |
| α$_{1A}$ | 45 | | Human |
| α$_{1B}$ | >10000 | | Human |
| α$_{2A}$ | 330 | | Human |
| α$_{2B}$ | 220 | | Human |
| α$_{2C}$ | 160 | | Human |
| β$_1$ | >10000 | | Human |
| β$_2$ | >10000 | | Human |
| H$_1$ | 0.06–2.3 | ↓ | Human |
| H$_2$ | 4.8 | | Human |
| H$_3$ | >10,000 | | Human |
| H$_4$ | 202–>10,000 | | Human |
| M$_1$ | 12 | ↓ | Human |
| M$_2$ | 7 | ↓ | Human |
| M$_3$ | 12 | ↓ | Human |
| M$_4$ | 8 | ↓ | Human |
| M$_5$ | 11.8 | ↓ | Human |
| I$_1$ receptor | 204 | | Human |
| σ$_1$ receptor | >10000 | | Guinea pig |
| σ$_2$ receptor | 750 | | Rat |
| SERTTooltip Serotonin transporter | >10000 | | Human |
| NETTooltip Norepinephrine transporter | 290–2550 | | Human |
| DATTooltip Dopamine transporter | 4100 | | Human |
| ↑ The smaller the equilibrium constant, the more strongly the drug binds to the site. ↑ ↑ Agonist↓ Antagonist |||||
Cyproheptadine is a very potent antihistamine or inverse agonist of the H$_1$ receptor. At higher concentrations, it also has anticholinergic, antiserotonergic, and antidopaminergic activities. Of the serotonin receptors, it is an especially potent antagonist of the 5-HT$_2$ receptors. This is thought to underlie its effectiveness in the treatment of serotonin syndrome. However, it is possible that blockade of 5-HT$_1$ receptors may also contribute to its effectiveness in serotonin syndrome. Cyproheptadine has been reported to block 85% of 5-HT$_2$ receptors in the human brain at a dose of 4 mg three times per day (12 mg/day total) and to block 95% of 5-HT$_2$ receptors in the human brain at a dose of 6 mg three times per day (18 mg/day total) as measured with positron emission tomography (PET). The dose of cyproheptadine recommended to ensure blockade of the 5-HT$_2$ receptors for serotonin syndrome is 20 to 30 mg. Besides its activity at neurotransmitter targets, cyproheptadine has been reported to possess weak antiandrogenic activity.

### Pharmacokinetics

Cyproheptadine is well-absorbed following oral ingestion, with peak plasma levels occurring after 1 to 3 hours. Its terminal half-life when taken orally is approximately 8 hours.

## Chemistry

Cyproheptadine is a tricyclic benzocycloheptene and is closely related to pizotifen and ketotifen as well as to tricyclic antidepressants.

## Research

Cyproheptadine was studied in one small trial as an adjunct in people with schizophrenia whose condition was stable and were on other medication; while attention and verbal fluency appeared to be improved, the study was too small to draw generalizations from. It has also been studied as an adjuvant in two other trials in people with schizophrenia, around fifty people overall, and did not appear to have an effect.
There have been some trials to see if cyproheptadine could reduce sexual dysfunction caused by SSRI and antipsychotic medications.
Cyproheptadine has been studied for the treatment of post-traumatic stress disorder.

## Veterinary use

Cyproheptadine is used in cats as an appetite stimulant$^{:1371}$ and as an adjunct in the treatment of asthma. Possible adverse effects include excitement and aggressive behavior. The elimination half-life of cyproheptadine in cats is 12 hours.
Cyproheptadine is a second line treatment for pituitary pars intermedia dysfunction in horses.