Bactrim Dosing for Cellulitis
For adults with normal renal function, the recommended dosing of trimethoprim-sulfamethoxazole (Bactrim) for cellulitis is 1-2 double-strength tablets (160mg/800mg) twice daily for 5-10 days. 1, 2
Antibiotic Selection for Cellulitis
Purulent Cellulitis
For purulent cellulitis (associated with purulent drainage or exudate):
- First-line therapy: Trimethoprim-sulfamethoxazole (TMP-SMX) 1-2 double-strength tablets (160mg/800mg) PO BID 1
- Alternative options:
- Clindamycin 300-450mg PO TID
- Doxycycline 100mg PO BID (not for children <8 years)
- Linezolid 600mg PO BID (for severe cases)
Non-purulent Cellulitis
For non-purulent cellulitis (no drainage or exudate):
- First-line therapy: Beta-lactam antibiotics (e.g., cephalexin 500mg PO QID)
- Alternative options (if MRSA suspected or beta-lactam allergy):
- Clindamycin 300-450mg PO TID
- TMP-SMX 1-2 double-strength tablets BID (though less effective as monotherapy against streptococci) 1
Duration of Therapy
The American College of Physicians and Infectious Diseases Society of America recommend a 5-6 day course of antibiotics for uncomplicated cellulitis in patients who can self-monitor and have close follow-up 1, 2.
Weight-Based Dosing Considerations
Research suggests that adequate weight-based dosing of TMP-SMX improves outcomes in cellulitis. For optimal efficacy, TMP-SMX should be dosed at ≥5mg/kg/day of the trimethoprim component 3. For a 70kg adult, this would equate to at least one double-strength tablet (160mg/800mg) twice daily.
Special Considerations
- MRSA prevalence: In areas with high MRSA prevalence, TMP-SMX has shown higher success rates compared to beta-lactams alone 4
- Combination therapy: Adding TMP-SMX to cephalexin has not consistently shown improved outcomes for uncomplicated cellulitis without abscess 5, 6
- Renal function: Dosage adjustment is necessary for patients with impaired renal function
- Pregnancy: TMP-SMX is pregnancy category C/D and not recommended for women in the third trimester of pregnancy 1
- Pediatrics: Not recommended for children <2 months of age 1
Monitoring and Follow-up
- Evaluate clinical response within 48-72 hours of initiating therapy
- Monitor for adverse effects including rash, gastrointestinal symptoms, and rare but serious reactions like Stevens-Johnson syndrome
- Consider culture and susceptibility testing for non-responding infections
When to Consider Alternative Therapy
Consider alternative antibiotics if:
- Patient has sulfa allergy
- Infection is severe or rapidly progressing
- Patient has significant comorbidities or immunosuppression
- No improvement after 48-72 hours of therapy
TMP-SMX is particularly effective against community-acquired MRSA but has less reliable activity against beta-hemolytic streptococci, which are common causes of non-purulent cellulitis 1.