What is the dosage and usage of Co-trimoxazole (trimethoprim/sulfamethoxazole) for treating bacterial infections?

Co-trimoxazole (Trimethoprim/Sulfamethoxazole) Dosage and Usage for Bacterial Infections

For bacterial infections, co-trimoxazole (trimethoprim/sulfamethoxazole) is typically dosed as 1-2 double-strength tablets (160mg/800mg) twice daily for adults, or 8-12 mg/kg/day (based on trimethoprim component) in divided doses for children. 1, 2

Adult Dosing Regimens by Infection Type

Skin and Soft Tissue Infections

• MRSA skin infections: 1-2 double-strength tablets twice daily PO 1
• Non-purulent cellulitis: 1-2 double-strength tablets twice daily PO 1
• Duration: 5-10 days for uncomplicated infections; 14-21 days for severe infections 3

Urinary Tract Infections

• Standard dosage: 1 double-strength tablet or 2 single-strength tablets every 12 hours 2
• Duration: 10-14 days 2
• Renal adjustment: For creatinine clearance 15-30 mL/min, reduce to half the usual regimen; not recommended for clearance below 15 mL/min 2

Respiratory Infections

• Acute exacerbations of chronic bronchitis: 1 double-strength tablet or 2 single-strength tablets every 12 hours 2
• Duration: 14 days 2

Pneumocystis jirovecii Pneumonia

• Treatment: 75-100 mg/kg/day sulfamethoxazole and 15-20 mg/kg/day trimethoprim in divided doses every 6 hours 2
• Duration: 14-21 days 2
• Prophylaxis (adults): 1 double-strength tablet daily 2

Other Infections

• Shigellosis: Same as UTI dosage but for 5 days 2
• Traveler's diarrhea: 1 double-strength tablet or 2 single-strength tablets every 12 hours for 5 days 2

Pediatric Dosing

• Age restriction: Contraindicated in children less than 2 months of age 2
• UTI/Otitis media/Shigellosis: 40 mg/kg sulfamethoxazole and 8 mg/kg trimethoprim per 24 hours, divided every 12 hours 2
• Weight-based dosing guide:
  • 10 kg: Half tablet every 12 hours
  • 20 kg: 1 tablet every 12 hours
  • 30 kg: 1½ tablets every 12 hours
  • 40 kg: 2 tablets or 1 DS tablet every 12 hours 2

Clinical Considerations and Cautions

Efficacy Considerations

• Co-trimoxazole is bactericidal but has limited published efficacy data for some infections 1
• For MRSA infections, co-trimoxazole is considered effective but may not be first-line in all situations 1
• For streptococcal infections, activity against β-hemolytic streptococci is not well-defined 1

Important Contraindications

• Pregnancy category C/D - not recommended for women in third trimester 1
• Children under 2 months of age 2
• Severe renal impairment (CrCl <15 mL/min) 2

Common Pitfalls to Avoid

1. Inadequate duration of therapy - premature discontinuation increases risk of relapse 3
2. Overlooking surgical drainage - for abscesses, surgical drainage is essential and may be sufficient for small lesions 3
3. Ignoring local resistance patterns - check local antibiograms before prescribing 3
4. Failing to adjust for renal function - dose reduction required for impaired renal function 2
5. Using for streptococcal infections without additional coverage - limited activity against β-hemolytic streptococci 1

Alternative Antibiotics When Co-trimoxazole Cannot Be Used

For skin infections when co-trimoxazole is contraindicated, consider:

• Clindamycin: 300-450 mg PO three times daily (adults) 1
• Doxycycline: 100 mg PO twice daily (adults) 1
• Linezolid: 600 mg PO twice daily (adults) 1

For complicated infections requiring IV therapy:

• Vancomycin: 15-20 mg/kg/dose IV every 8-12 hours 1
• Linezolid: 600 mg IV/PO twice daily 1
• Daptomycin: 4 mg/kg IV once daily 1

Remember that proper specimen collection for culture before starting antibiotics and surgical drainage of purulent lesions are essential components of management for many bacterial infections 1, 3.