Adverse Effects of TMP-SMX
TMP-SMX causes dermatologic and hematologic adverse effects most commonly, with serious reactions being rare but potentially fatal, including Stevens-Johnson syndrome, toxic epidermal necrolysis, blood dyscrasias, and electrolyte disturbances—particularly hyperkalemia and hyponatremia.
Most Common Adverse Effects
Dermatologic Reactions
- Mild cutaneous reactions (erythematous maculopapular rash) occur in approximately 16% of children and 13% of adults receiving high-dose therapy 1, 2
- Severe skin reactions are rare but life-threatening:
- Drug must be permanently discontinued if urticarial rash or Stevens-Johnson syndrome occurs 3, 4
Hematologic Abnormalities
The incidence varies widely (0.1% to 34%) depending on patient population and dose 1:
- Leukopenia and thrombocytopenia are most common 1
- Severe blood dyscrasias include:
- Hemolytic anemia can occur in G6PD-deficient patients, though rarely reported 1
- Fatal hematologic reactions have been documented, though extremely rare 5
Serious Metabolic and Electrolyte Disturbances
Hyperkalemia
- Significantly increased risk with high-dose therapy (3.46% vs 0.81% with standard dose) 2
- Risk factors include:
- 10-fold increased reporting compared to azithromycin 6
Hyponatremia
- Uncommon but potentially life-threatening 7
- Can occur acutely and cause lethargy 7
- 10-fold increased reporting compared to azithromycin 6
Organ-Specific Toxicity
Renal Effects
- Acute kidney injury occurs in 3.67% with high-dose vs 1.63% with standard dose 2
- Interstitial nephritis reported 3, 4
- Crystalluria can occur without adequate hydration 4
- Serum creatinine elevation is an independent risk factor for mortality in ICU patients 8
Hepatic Effects
- Hepatitis and elevated transaminases 3
- Fulminant hepatic necrosis (rare but fatal) 4
- ALT elevation associated with concomitant teicoplanin use 8
Gastrointestinal Effects
- Mild GI complaints are common 3
- Clostridium difficile-associated diarrhea (CDAD) can occur, ranging from mild to fatal colitis 4
Population-Specific Considerations
HIV/AIDS Patients
- 40-65% of adults with AIDS experience adverse reactions vs 15% of HIV-infected children 1
- Higher incidence of:
Elderly Patients
- Increased risk of severe adverse reactions, particularly with comorbidities 4
- Thrombocytopenia with purpura more common, especially with concurrent thiazide diuretics 4
- Folic acid deficiency effects more likely 4
Neonates
- Contraindicated in infants <2 months due to bilirubin displacement and immature drug metabolism 1
Rare but Serious Adverse Effects
- Fatal reactions: <1/100,000 children 1
- Hypoglycemia in non-diabetic patients (rare), especially with renal dysfunction, liver disease, or malnutrition 4
- Neuropsychiatric effects: Acute psychosis, hallucinations (very rare) 9
- Metabolic acidosis: Associated with mortality in ICU patients 8
- Pancytopenia: 10-fold increased reporting vs azithromycin 6
Critical Drug Interactions
- Phenytoin: Increased half-life by 39% 4
- Methotrexate: Increased free concentrations; severe toxicity possible 4
- Warfarin: Potentiated anticoagulant effect 4
- Oral hypoglycemics: Potentiated effect 4
- Digoxin: Increased blood levels, especially in elderly 4
- Cyclosporine: Marked but reversible nephrotoxicity 4
Monitoring Recommendations
Complete blood counts with differential and platelet count should be performed:
Electrolyte monitoring is essential in:
Renal function tests and urinalyses should be performed during therapy, particularly in patients with impaired renal function 4.