Cotrimoxazole (800/160 mg) Dosing Recommendations
Cotrimoxazole 800/160 mg (sulfamethoxazole/trimethoprim) should be taken as one double-strength tablet twice daily for 3 days for acute uncomplicated cystitis, or twice daily for 5 days for shigellosis and traveler's diarrhea. 1, 2, 3
Standard Dosing by Indication
Urinary Tract Infections (Acute Uncomplicated Cystitis)
- Dose: 1 double-strength tablet (800/160 mg) twice daily for 3 days 1
- Critical caveat: Only use if local resistance rates are <20% or if the organism is known to be susceptible 1
- Clinical cure rates reach 90-100% when organisms are susceptible, but drop dramatically to 41-54% with resistant organisms 1
Shigellosis and Traveler's Diarrhea
Acute Exacerbations of Chronic Bronchitis
Pneumocystis jirovecii Pneumonia (PCP)
- Treatment dose: 75-100 mg/kg/day sulfamethoxazole (15-20 mg/kg/day trimethoprim) divided every 6 hours for 14-21 days 2, 3
- This translates to approximately 2-3 double-strength tablets every 6 hours for a 70 kg adult
- Prophylaxis dose: 1 double-strength tablet daily 2, 3
- Important update: Recent evidence suggests lower doses (≤10 mg/kg/day trimethoprim) may be equally effective with significantly less hyperkalemia (HR 0.60) compared to high-dose therapy, though 30-day mortality was not different 4
Renal Dosing Adjustments
Critical for preventing toxicity, particularly trimethoprim accumulation:
- CrCl >30 mL/min: Standard dosing 2, 3
- CrCl 15-30 mL/min: Reduce dose by 50% (½ the usual regimen) 2, 3
- CrCl <15 mL/min: Contraindicated - do not use 2, 3
Key pharmacokinetic consideration: Trimethoprim accumulates significantly when eGFR <60 mL/min/1.73 m², creating overdose risk, while sulfamethoxazole concentrations paradoxically decrease, risking underdosing 5. For patients with eGFR 30 mL/min/1.73 m², reduce dose by 16.7%; for eGFR 10 mL/min/1.73 m², reduce by 33.3% 6.
Special Populations
Continuous Renal Replacement Therapy (CRRT)
- Sulfamethoxazole clearance increases on CRRT, but trimethoprim clearance does not 5, 6
- Consider doubling the sulfamethoxazole component to maintain adequate exposure 6
- The fixed-dose combination creates a therapeutic mismatch in this setting 5
Obese Patients
- No dose adjustment needed based on weight alone for standard infections 5
- Weight influences sulfamethoxazole but not trimethoprim pharmacokinetics 5
Administration Recommendations
- Take with food or milk to minimize gastrointestinal upset
- Maintain adequate hydration (drink full glass of water with each dose)
- Space doses evenly (every 12 hours for twice-daily dosing)
- Can be taken without regard to meals, though food may improve tolerability
Common Pitfalls to Avoid
Resistance blindness: Do not use empirically if local E. coli resistance exceeds 20% - efficacy drops from 88% to 54% 1
Renal function neglect: Failure to dose-adjust for CrCl 15-30 mL/min leads to trimethoprim toxicity, particularly hyperkalemia (occurs in 27.4% of patients) 4, 6
Duration errors: Using 7-10 day courses when 3 days is equally effective for uncomplicated cystitis increases adverse effects from 4% to 24% without improving outcomes 7
CRRT miscalculation: Assuming both components are equally dialyzed - only sulfamethoxazole is significantly removed 5, 6
Comparative Context
While cotrimoxazole remains highly effective when organisms are susceptible (90-95% cure rates) 1, nitrofurantoin is now preferred as first-line therapy for acute uncomplicated cystitis due to minimal resistance and lower collateral damage, despite equivalent efficacy 1. Cotrimoxazole should be reserved for situations where nitrofurantoin cannot be used or when susceptibility is confirmed.