Postoperative Cephalexin Dosing
For uncomplicated clean surgery in adults with normal renal function, discontinue cephalexin prophylaxis within 24 hours after the procedure; if a documented postoperative infection develops, treat with cephalexin 500 mg orally every 6 hours (four times daily) for 7–10 days. 1
Duration of Surgical Antibiotic Prophylaxis
Stop prophylaxis at 24 hours postoperatively for uncomplicated clean (Altemeier class I) procedures—extending beyond this window increases antimicrobial resistance and exposes patients to adverse effects including Clostridioides difficile infection without improving wound healing. 1
The absolute maximum duration is 48 hours, reserved exclusively for high-risk cases such as cardiac surgery with extracorporeal circulation or prosthetic joint implantation. 1
Intraoperative redosing is required only if the operation exceeds approximately 4 hours (two half-lives of cephalexin/cefazolin); otherwise a single preoperative dose suffices. 1
When to Transition from Prophylaxis to Treatment
Discontinue prophylaxis if the wound shows appropriate healing and no purulent drainage by postoperative day 1. 1
Switch to therapeutic dosing (500 mg every 6 hours for 7–10 days) if you document methicillin-susceptible Staphylococcus aureus (MSSA) or streptococcal infection with systemic inflammatory response syndrome (SIRS) criteria: temperature >38°C or <36°C, heart rate >90 bpm, respiratory rate >24/min, or white blood cell count >12,000 or <4,000 cells/µL. 1
Therapeutic Dosing for Documented Postoperative Infection
Adults with normal renal function: Cephalexin 500 mg orally every 6 hours (four times daily) for skin and soft-tissue infections caused by MSSA. 2, 1, 3
Avoid three-times-daily dosing (500 mg TID)—cephalexin's short half-life (approximately 1 hour) requires every-6-hour administration to maintain effective tissue concentrations. 1
For severe infections requiring >4 g/day, switch to parenteral cefazolin 1 g intravenously every 8 hours rather than escalating oral cephalexin. 1, 3
Infection-Specific Considerations
Surgery of Trunk or Extremity (Away from Axilla/Perineum)
- Cephalexin 500 mg every 6 hours orally provides adequate coverage for MSSA and streptococcal species in clean surgical sites. 2
Surgery of Axilla or Perineum
- Do not use cephalexin monotherapy—these sites require anaerobic and gram-negative coverage (e.g., metronidazole 500 mg every 8 hours plus ciprofloxacin 750 mg every 12 hours orally). 2
Diabetic Foot or Polymicrobial Wounds
- Cephalexin is inappropriate—select amoxicillin-clavulanate or a broader-spectrum regimen to cover gram-negative organisms and anaerobes. 1
Bite-Related Cellulitis
- Cephalexin lacks coverage for Pasteurella multocida (animal bites) and Eikenella corrodens (human bites); use amoxicillin-clavulanate instead. 1
When Cephalexin Is Not Appropriate
MRSA suspected or confirmed: Switch to trimethoprim-sulfamethoxazole (1–2 double-strength tablets twice daily) or clindamycin (300–450 mg every 6 hours). 1
Severe penicillin allergy (anaphylaxis, angioedema, urticaria): Cephalexin is contraindicated due to cross-reactivity; use a fluoroquinolone or clindamycin. 1, 4
Non-severe penicillin allergy (delayed rash): Cephalexin may be used cautiously. 1
Renal Dose Adjustment
Creatinine clearance <30 mL/min: Reduce dosing frequency proportionally (e.g., 500 mg every 12 hours instead of every 6 hours) while maintaining the milligram dose per administration. 5, 6
Hemodialysis: Administer cephalexin after dialysis sessions, as 58% of the drug is removed during a 6-hour dialysis run. 7
Common Pitfalls
Extending prophylaxis beyond 24 hours "just to be safe" increases resistance and C. difficile risk without benefit. 1
Underdosing with 250 mg every 6 hours for documented infections—use 500 mg every 6 hours to achieve adequate tissue penetration. 1, 3
Using cephalexin for necrotizing fasciitis or deep abscesses—these require broader coverage (e.g., piperacillin-tazobactam or vancomycin plus a carbapenem). 2