Cephalexin
Brand names: Keflex
Class: 💉 Penicillins, Cephalosporins & UTI Antibiotics
The NP-board angle on cephalexin is non-purulent skin/soft-tissue infection and the penicillin-allergy nuance. As a first-generation cephalosporin, it covers gram-positives — particularly Staph aureus (MSSA) and group A strep — making it first-line for non-purulent cellulitis. It is also an option for strep pharyngitis when penicillin is intolerable, for uncomplicated UTI in pregnancy, and for dental endocarditis prophylaxis when penicillin allergy is non-severe. Cross-reactivity with penicillin is roughly 1–2% and lowest with later-generation cephalosporins; avoid all beta-lactams if there is a documented anaphylactic reaction to penicillin. For purulent SSTI suspicious for CA-MRSA, switch to TMP-SMX or doxycycline.
✅ Indications
Cellulitis/SSTI, UTI in pregnancy, strep pharyngitis alternative, dental prophylaxis (if PCN allergy non-severe).
⚙️ Mechanism of Action
1st-generation cephalosporin — cell wall inhibitor.
📏 Dosing
250–500 mg PO QID or 500 mg BID–TID.
🚫 Contraindications
Cephalosporin allergy, severe PCN allergy (anaphylaxis).
⚠️ Adverse Effects
GI upset, rash, C. diff, elevated LFTs.
🔬 Monitoring
Clinical response.
💎 Board Pearls
- 🦠 First-line outpatient cellulitis (covers streptococci + MSSA).
- 🤰 Pregnancy-safe UTI option (with amoxicillin, nitrofurantoin).
- ⚠️ Cross-reactivity with PCN is LOW (<1%) — document carefully; many mislabeled allergies.
Practice Questions
Which of the following is the first-line oral antibiotic for an otherwise healthy adult with uncomplicated, non-purulent cellulitis of the lower extremity who has no MRSA risk factors and no penicillin allergy?
Related Drugs in This Class
- Amoxicillin — Amoxil
- Amoxicillin-Clavulanate — Augmentin
- Trimethoprim-Sulfamethoxazole — Bactrim, Septra, TMP-SMX
- Nitrofurantoin — Macrobid, Macrodantin
Sources
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