What is the recommended dose of Bactrim (trimethoprim-sulfamethoxazole) for an adult patient with a skin or soft tissue abscess?

Bactrim Dosing for Skin Abscess

For an adult with a skin abscess, prescribe Bactrim (trimethoprim-sulfamethoxazole) 1-2 double-strength tablets (160mg/800mg) twice daily for 7-10 days after incision and drainage. 1, 2

Primary Treatment Approach

Incision and drainage is the cornerstone of abscess management and must be performed first. 1 The decision to add antibiotics depends on specific clinical features outlined below.

When to Add Antibiotics After Drainage

Add Bactrim after incision and drainage if any of the following are present: 1

• Multiple sites of infection or extensive disease
• Rapid progression or associated cellulitis
• Systemic signs of illness (fever, tachycardia)
• Comorbidities (diabetes, HIV/AIDS, immunosuppression)
• Extremes of age
• Difficult-to-drain locations (face, hand, genitalia)
• Lack of response to drainage alone

For simple, small abscesses (<5 cm) in healthy patients without these features, incision and drainage alone may be adequate. 1, 3

Specific Dosing Regimen

Standard Adult Dose

• 1-2 double-strength tablets (800mg sulfamethoxazole/160mg trimethoprim) orally twice daily 1, 2
• Duration: 7-10 days (10 days preferred to reduce recurrence) 2, 3

Pediatric Dose (if applicable)

• Trimethoprim 4-6 mg/kg/dose, sulfamethoxazole 20-30 mg/kg/dose orally every 12 hours 1
• Contraindicated in children <2 months of age 2

Evidence Supporting This Recommendation

The highest quality recent trial demonstrated that Bactrim after incision and drainage improved cure rates from 69% (placebo) to 82% compared to drainage alone in patients with abscesses ≤5 cm. 3 This benefit was specifically seen in patients with confirmed S. aureus infection, including MRSA. 3

Importantly, Bactrim also reduced new lesion formation at 30 days (9% vs 28% with placebo), which is clinically significant for preventing recurrent infections. 3, 4

Critical Coverage Considerations

MRSA vs Streptococcal Coverage

• Bactrim has excellent activity against CA-MRSA (the most common pathogen in purulent abscesses) 1, 2
• Bactrim has poor activity against β-hemolytic streptococci 1, 2

For purulent cellulitis or abscesses (pus present), Bactrim monotherapy is appropriate. 1, 2

For non-purulent cellulitis (no pus), add a β-lactam like amoxicillin 500mg three times daily to cover streptococci, or use clindamycin alone instead. 1, 2

Dosing Nuances: Standard vs High Dose

A prospective study found no difference in clinical resolution between standard dose (160/800mg twice daily) and high dose (320/1600mg twice daily) for MRSA skin infections. 5 Therefore, use the standard dose of 1-2 double-strength tablets twice daily to minimize side effects while maintaining efficacy. 1, 2, 5

Important Safety Warnings

• Pregnancy Category C/D: Avoid in third trimester 1, 2
• Monitor for sulfa allergy (consider clindamycin or doxycycline as alternatives) 2
• Severe drug interactions with methotrexate (can cause bone marrow suppression) 2
• Adverse events occur in 11% of patients (less than clindamycin at 22%) but typically resolve without sequelae 3

Alternative Agents if Bactrim is Contraindicated

If Bactrim cannot be used: 1, 2

• Clindamycin 300-450mg orally three times daily (covers both MRSA and streptococci, but higher C. difficile risk)
• Doxycycline 100mg orally twice daily (MRSA coverage, poor streptococcal coverage, avoid in children <8 years)
• Minocycline 200mg once, then 100mg twice daily (may be more effective than Bactrim or doxycycline for treatment failures) 6

Common Pitfalls to Avoid

• Do not use Bactrim for non-purulent cellulitis without adding streptococcal coverage 1, 2
• Do not skip incision and drainage – antibiotics alone are insufficient 1
• Do not use high-dose Bactrim routinely – standard dosing is equally effective with fewer side effects 5
• Do not use for animal/human bites – poor anaerobic coverage; use amoxicillin-clavulanate instead 2