Frequency of Adverse Reactions to Bactrim
Adverse reactions to Bactrim (trimethoprim-sulfamethoxazole) occur in approximately 6-8% of patients in the general population, with the most common being mild gastrointestinal disturbances and skin reactions. 1
Common Adverse Reactions (Most Frequent)
- Gastrointestinal disturbances are the most common adverse reactions, including nausea, vomiting, and anorexia 2
- Allergic skin reactions such as rash and urticaria occur frequently 2
- Gastrointestinal intolerance and skin eruptions are the most prevalent adverse reactions overall 3
Serious but Rare Adverse Reactions
While most adverse reactions are mild, serious toxicity is a rare event 4. When severe reactions do occur, they typically manifest as:
- Severe cutaneous adverse reactions (SCARs) including Stevens-Johnson syndrome, toxic epidermal necrolysis, and drug reaction with eosinophilia and systemic symptoms (DRESS) 2
- Hematologic toxicity including thrombocytopenia, leukopenia, agranulocytosis, and aplastic anemia 2, 4
- Fulminant hepatic necrosis 2
- Anaphylaxis and circulatory shock 2
Special Population Considerations
AIDS Patients - Dramatically Higher Risk
The incidence of adverse reactions is greatly increased in AIDS patients compared to non-AIDS patients, particularly when treating Pneumocystis carinii pneumonia 5:
- Rash, fever, leukopenia, and elevated liver enzymes occur at substantially higher rates 5
- Hyperkalemia appears to be increased in AIDS patients receiving Bactrim 5
- Adverse effects are generally less severe when used for prophylaxis rather than active treatment 5
Timing of Adverse Reactions
- Most untoward reactions develop within two weeks of onset of therapy 3
- The incidence compares favorably with standard agents used for the same indications 3
Risk Factors for Increased Toxicity
Patients at higher risk for adverse reactions include those with 5:
- Impaired renal or hepatic function
- Possible folate deficiency (elderly, chronic alcoholics, patients on anticonvulsants, malabsorption syndrome, malnutrition)
- Severe allergies or bronchial asthma
- Glucose-6-phosphate dehydrogenase deficiency (risk of hemolysis)
- Renal dysfunction, liver disease, or malnutrition (higher risk of hypoglycemia)
Clinical Pitfall
Rare severe allergic reactions, particularly Stevens-Johnson syndrome, make Bactrim a potentially undesirable therapy when alternative treatments exist 6. This is especially relevant when considering off-label uses where other effective options are available.