Thrombocytopenia has been reported with both sulfamethoxazole and trimethoprim usage. A case control study found a 124-fold increased risk of severe thrombocytopenia (platelets <30,000 µL, requiring hospitalization) with sulfamethoxazole and trimethoprim in combination with an incidence of 2 cases per 1000 patient-years of exposure.6 The incidence of less severe thrombocytopenia may be higher.

Thrombocytopenia caused by sulfamethoxazole and trimethoprim may be immune-mediated. Drug-induced immune-mediated thrombocytopenia with sulfamethoxazole/trimethoprim is characterized by a drug-dependent antibody that is itself nonreactive, but when soluble drug is present at pharmacologic concentrations, antibody binds tightly to specific platelet membrane glycoproteins, causing platelet destruction.7 Serologic testing for drug-specific antibody is commercially available and may be useful for identifying the specific cause of thrombocytopenia in individual cases. Testing is important because a patient with drug-dependent antibodies should not be re-exposed to BACTRIM (see CONTRAINDICATIONS).

Typically, a patient with immune thrombocytopenia will have taken drug for about 1 week or intermittently over a longer period of time (possibly years) before presenting with petechiae or bruising. Systemic symptoms, such as lightheadedness, chills, fever, nausea, and vomiting, often may precede bleeding events. Thrombocytopenia may be severe. Patients should have risk/benefit re-evaluated in order to continue treatment with BACTRIM. If the drug is stopped, symptoms usually resolve within 1 or 2 days and platelet count returns to normal in less than 1 week. If BACTRIM is not stopped, there is a risk of fatal hemorrhage. The onset of thrombocytopenia may be more rapid upon re-exposure.

Sulfamethoxazole has also been shown to occasionally trigger the production of platelet-specific autoantibodies leading to a clinical picture indistinguishable from spontaneous autoimmune thrombocytopenia. In such cases, autoantibodies remain present for up to 9 weeks after sulfamethoxazole has been stopped, corresponding with a marked reduction in platelet counts for the same period.

The sulfonamides should not be used for treatment of group A β-hemolytic streptococcal infections. In an established infection, they will not eradicate the streptococcus and, therefore, will not prevent sequelae such as rheumatic fever.

Clostridium difficile associated diarrhea (CDAD) has been reported with use of nearly all antibacterial agents, including BACTRIM, and may range in severity from mild diarrhea to fatal colitis. Treatment with antibacterial agents alters the normal flora of the colon leading to overgrowth of C. difficile.

C. difficile produces toxins A and B which contribute to the development of CDAD. Hypertoxin producing strains of C. difficile cause increased morbidity and mortality, as these infections can be refractory to antimicrobial therapy and may require colectomy. CDAD must be considered in all patients who present with diarrhea following antibiotic use. Careful medical history is necessary since CDAD has been reported to occur over two months after the administration of antibacterial agents.

If CDAD is suspected or confirmed, ongoing antibiotic use not directed against C. difficile may need to be discontinued. Appropriate fluid and electrolyte management, protein supplementation, antibiotic treatment of C. difficile, and surgical evaluation should be instituted as clinically indicated.