Cotrimoxazole Coverage and Dosing
Cotrimoxazole (trimethoprim-sulfamethoxazole) provides excellent coverage against Staphylococcus aureus including MRSA, but has poor activity against Group A Streptococcus and anaerobes, requiring combination therapy with a beta-lactam when streptococcal infection is suspected. 1, 2
Antimicrobial Spectrum
Organisms Covered
- Staphylococcus aureus (including community-acquired MRSA strains) - this is the primary indication 1, 2
- β-hemolytic streptococci - covered but NOT first-line therapy 2
- Gram-negative organisms including E. coli, Klebsiella, and Proteus mirabilis in urinary tract infections 3, 4
- Pneumocystis jirovecii (formerly P. carinii) 3
Critical Coverage Gaps
- Poor activity against Group A Streptococcus - do not use as monotherapy for non-purulent cellulitis 1, 5
- No anaerobic coverage - inadequate for polymicrobial or anaerobic infections 1
- Increasing MRSA resistance reported in some regions 2
Standard Dosing Regimens
Adults - Skin and Soft Tissue Infections
- 1-2 double-strength tablets (800mg sulfamethoxazole/160mg trimethoprim) twice daily 1, 2, 5
- Duration: 7-14 days based on clinical response 2, 5
- For severe infections requiring IV therapy: 8-12 mg/kg/day (based on trimethoprim component) divided into 4 doses 5
Pediatric Dosing (>2 months of age)
- 8-12 mg/kg/day (based on trimethoprim component) in 2 divided doses orally 5, 3
- NOT recommended for infants under 2 months of age 5, 3
Urinary Tract Infections
- Adults: 1 double-strength tablet every 12 hours for 10-14 days for complicated UTI 3
- Uncomplicated UTI: 3-day courses are effective with bacteriological cure rates of 86-87% 4
- Single-dose therapy (1.92-2.88g) is effective for uncomplicated UTI but one-day treatment (twice daily dosing) has fewer side effects 6, 7
Pneumocystis Pneumonia
- Treatment: 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours divided every 6 hours for 14-21 days 3
- Prophylaxis (adults): 1 double-strength tablet daily 3
Clinical Applications by Infection Type
Purulent Skin Infections (Abscesses, Furuncles, Carbuncles)
- First-line oral agent when MRSA is suspected or confirmed 1, 2
- Incision and drainage remains primary intervention; antibiotics are adjunctive 1
- Treatment duration: 7-14 days, with 10-day courses superior to 3-day courses for reducing recurrence 5
Non-Purulent Cellulitis
- Must be combined with a beta-lactam (cephalexin, amoxicillin, or dicloxacillin) to cover streptococci 1, 5
- Beta-lactams alone are preferred when streptococci are the likely pathogens 5
Wound Infections
- Effective for MRSA coverage but requires combination with anaerobic coverage for polymicrobial wounds 5
- For mild infections after drainage, may not need antibiotics at all 1
Critical Precautions and Contraindications
Pregnancy
Renal Impairment
- CrCl >30 mL/min: Standard dosing 3
- CrCl 15-30 mL/min: Reduce dose by 50% 3
- CrCl <15 mL/min: Use not recommended 3
Serious Adverse Effects
- Stevens-Johnson syndrome and toxic epidermal necrolysis - monitor for rash 1
- Bone marrow suppression 1
- Gastrointestinal disturbances and photosensitivity are common 1
Common Clinical Pitfalls
When NOT to Use Cotrimoxazole Alone
- Never use as monotherapy for suspected streptococcal cellulitis - add a beta-lactam 1, 5
- Avoid in superficial impetigo - topical mupirocin 2% is preferred to avoid systemic antibiotic exposure 5
- Inadequate for mixed aerobic-anaerobic infections without additional anaerobic coverage 5
Resistance Considerations
- Approximately 24% of E. coli strains show in vitro resistance to cotrimoxazole in some regions 8
- Local resistance patterns should guide empiric therapy decisions 2