Recommended Dosage for Cotrimoxazole (Sulfamethoxazole/Trimethoprim)
The standard adult dosage for cotrimoxazole is one double-strength tablet (800 mg sulfamethoxazole/160 mg trimethoprim) twice daily for most infections, with dosage adjustments required based on indication, age, weight, and renal function. 1, 2
Adult Dosing by Indication
- Urinary Tract Infections: One double-strength tablet (800 mg/160 mg) or two single-strength tablets (400 mg/80 mg) every 12 hours for 10-14 days 1
- Acute Exacerbations of Chronic Bronchitis: One double-strength tablet or two single-strength tablets every 12 hours for 14 days 1
- Traveler's Diarrhea: One double-strength tablet or two single-strength tablets every 12 hours for 5 days 1
- Shigellosis: Same as UTI dosing but for 5 days duration 1
Pneumocystis jirovecii Pneumonia (PJP)
Treatment
- Adults and Children: 75-100 mg/kg/day sulfamethoxazole and 15-20 mg/kg/day trimethoprim, divided into equal doses every 6 hours for 14-21 days 1, 2
Prophylaxis
- Adults: One double-strength tablet daily 1, 2
- Alternative Adult Regimen: One double-strength tablet three times weekly (typically Monday-Wednesday-Friday) 3
- Children: 750 mg/m²/day sulfamethoxazole with 150 mg/m²/day trimethoprim given in divided doses twice daily, on 3 consecutive days per week 1, 2
Pediatric Dosing
- Children ≥2 months: 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim, divided into two doses every 12 hours 1, 2
- Weight-based dosing guide:
- Not recommended for infants <2 months of age 1, 2
Renal Dosage Adjustments
- Creatinine clearance >30 mL/min: Standard dosing regimen 1, 2
- Creatinine clearance 15-30 mL/min: Half the usual regimen 1, 2
- Creatinine clearance <15 mL/min: Use not recommended 1, 2
Important Clinical Considerations
- Higher doses of trimethoprim (250 mg twice daily) may be more effective than standard doses for complicated UTIs, though co-trimoxazole still appears to have better efficacy in these cases 4
- Single-day or single-dose treatment regimens have shown comparable cure rates to 7-day regimens for uncomplicated UTIs in women (82-87% cure rates) 5
- Elderly patients may require dose adjustment as they demonstrate higher peak concentrations and larger area under the curve for trimethoprim, with reduced renal clearance compared to younger adults 6
- Patient compliance is significantly better with once-daily regimens compared to twice-daily dosing (97.5% vs 79.1% compliance) 7
- When used for PJP prophylaxis in HIV patients, treatment should be initiated when CD4+ T-cell counts fall below 200 cells/μL 3
Drug Interactions
- Potential interaction between methotrexate and cotrimoxazole when dosed at 800 mg/160 mg twice daily; the prophylactic dose is generally better tolerated when used with methotrexate 8
- Dosage adjustments may be needed when co-administered with certain antiretrovirals 8
Remember that cotrimoxazole is relatively nontoxic in patients without AIDS but requires careful monitoring for adverse effects including rash, pruritus, cytopenias, and transaminase elevations 3, 9.