Keflex 1000 mg BID is NOT a suitable regimen for MSSA skin abscess after I&D
For MSSA skin abscesses after incision and drainage, antibiotics may not be necessary at all, but if used, the appropriate cephalexin dose is 500 mg every 6 hours (QID), not 1000 mg twice daily (BID).
When Antibiotics Are Actually Needed
The Infectious Diseases Society of America recommends adding antibiotic therapy to incision and drainage only for abscesses with:
- Severe or extensive disease 1
- Signs of systemic illness 1
- Comorbidities or immunosuppression 1
- Extremes of age 1
- Difficult-to-drain locations 1
- Associated septic phlebitis 1
- Lack of response to incision and drainage alone 1
A landmark randomized controlled trial demonstrated that antibiotics may be unnecessary after surgical drainage of uncomplicated skin abscesses, with a 90.5% cure rate in the placebo arm versus 84.1% in the cephalexin arm (no significant difference, P = 0.25). 2
The Dosing Problem with Your Proposed Regimen
The FDA-approved dosing for cephalexin in skin and skin structure infections is:
- Standard dose: 250 mg every 6 hours (QID) 3
- Alternative for certain infections: 500 mg every 12 hours (BID) for streptococcal pharyngitis, skin and skin structure infections, and uncomplicated cystitis 3
- More severe infections: larger doses may be needed, but if >4 g daily is required, parenteral cephalosporins should be considered 3
Your proposed regimen of 1000 mg BID (2000 mg total daily) falls into an awkward middle ground—it's higher than the standard 500 mg BID but administered on a BID schedule that may not provide optimal coverage for serious infections.
The MRSA Coverage Gap
The critical issue is that most community-acquired skin abscesses are now caused by MRSA, not MSSA. In the randomized trial, 87.8% of S. aureus isolates were MRSA 2. Cephalexin has no reliable activity against MRSA, despite paradoxically good clinical outcomes in some studies 4, 2.
If you have confirmed MSSA (not just assumed), cephalexin can be used, but:
- Penicillinase-resistant penicillins (flucloxacillin, dicloxacillin) remain the antibiotics of choice for serious MSSA infections 5
- First-generation cephalosporins like cephalexin have important therapeutic roles in less serious MSSA infections 5
Recommended Approach for MSSA Skin Abscess After I&D
If antibiotics are truly indicated (based on criteria above):
For uncomplicated MSSA skin abscess: Cephalexin 500 mg every 12 hours (BID) for 5-10 days 1, 3
For more severe MSSA infections: Cephalexin 500 mg every 6 hours (QID) 3, 5
If MRSA cannot be excluded (which is most cases): Use TMP-SMX 1-2 double-strength tablets twice daily or doxycycline 100 mg twice daily for 5-10 days 1
Critical Pitfalls to Avoid
- Do not assume MSSA without culture confirmation—most community skin abscesses are MRSA 2
- Do not use clindamycin empirically if local resistance rates are >10% 1
- Do not add rifampin to cephalexin—resistance develops rapidly without proven benefit 1
- Do not continue antibiotics beyond 5-10 days for uncomplicated abscesses 1
Bottom Line
The 1000 mg BID dosing is not FDA-approved and lacks supporting evidence. If you're treating confirmed MSSA and antibiotics are indicated, use either 500 mg BID (for uncomplicated infections) or 500 mg QID (for more severe infections). However, for most skin abscesses after adequate I&D, antibiotics may not be necessary at all 2.