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First-Generation Cephalosporin — Skin / Soft Tissue / UTI Pregnancy: Safe — first-generation cephalosporins considered safe throughout pregnancy

Cefalexin

Brand names: Ceporex, Keflex

Adult dose

Dose: 250–500 mg every 6 hours; UTI: 500 mg every 12 hours; Severe infection: 1–1.5 g every 6–8 hours
Route: Oral
Frequency: 2–4 times daily (indication-dependent)
Max: 4 g/day
Oral first-generation cephalosporin. Good MSSA activity — alternative to flucloxacillin for cellulitis step-down. Active against most gram-positive cocci and some gram-negatives (E. coli, Klebsiella, Proteus). Not active against MRSA, Pseudomonas, Enterococcus.

Paediatric dose

Dose: 12.5–25 mg/kg mg/kg
Route: Oral
Frequency: 4 times daily
Max: Adult dose
BNFc: available as 125 mg/5 mL oral suspension; widely used in paediatric skin/UTI infections

Dose adjustments

Renal

CrCl <30 mL/min: reduce dose or extend interval; CrCl <10: 250 mg every 8–12 hours

Hepatic

No adjustment required

Paediatric weight-based calculator

BNFc: available as 125 mg/5 mL oral suspension; widely used in paediatric skin/UTI infections

Clinical pearls

  • Cross-reactivity with penicillin: true allergy cross-reactivity is 1–2% (not 10% as historically claimed); skin testing recommended before use if penicillin anaphylaxis history — can usually be given safely if penicillin allergy is mild (rash only)
  • Preferred oral agent for step-down from IV flucloxacillin in MSSA cellulitis when flucloxacillin compliance poor (cefalexin with food vs flucloxacillin empty stomach)
  • Not effective against MRSA — send swabs before starting in cellulitis; MRSA decolonisation protocol if carrier
  • Take with or without food — food reduces peak concentration but not overall absorption

Contraindications

  • Cephalosporin allergy
  • Penicillin allergy with history of anaphylaxis (cross-reactivity ~1%)

Side effects

  • Diarrhoea (common)
  • Nausea
  • Rash
  • Hypersensitivity
  • C. difficile-associated diarrhoea
  • Positive Coombs test (haemolytic anaemia — rare)

Interactions

  • Warfarin — enhanced anticoagulant effect
  • Probenecid — increases cefalexin levels
  • Metformin — minor renal tubular competition

Monitoring

  • Renal function
  • C. difficile if prolonged course or diarrhoea develops
  • Rash (penicillin allergy concern)

Reference: BNFc; BNF 90; NICE CG121 (Cellulitis); PHE Antibiotic Guidelines; UK Allergy Guidelines (Penicillin Cross-Reactivity). Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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