Cephalosporins · Anti-infectives
Cephalexin
cefalexin · Keflex®(brand names vary by market)
The oral first-generation workhorse for skin and soft tissue.
Also: tablet, oral liquid
Mechanism
Binds penicillin-binding proteins and blocks the final cross-linking step of peptidoglycan synthesis, leaving the cell wall unable to withstand osmotic pressure.
Indications
- Cellulitis and other skin and soft tissue infection
- Streptococcal pharyngitis
- Uncomplicated urinary tract infection
- Bone and joint infection as step-down
Formulations
Dosing concepts
Time above MIC
Beta-lactams are time-dependent killers, so frequent dosing matters more than a large single dose. Reducing frequency to improve adherence undermines the pharmacology.
Renal impairment
Renally cleared; the interval is extended as clearance falls.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- ~90%
- Half-life
- ~1 h
- Elimination
- renal: Excreted largely unchanged
- Protein binding
- ~15%
Adverse effects
- Diarrhoeacommon
- Clostridioides difficile infectionserious
- Rashcommon
- AnaphylaxisseriousRare; cross-reactivity with penicillin is far lower than the old 10 percent figure suggested.
Contraindications & precautions
Contraindications
- Prior severe immediate hypersensitivity to a cephalosporin
Precautions
- Prior penicillin anaphylaxis
- Renal impairment
- Prior C. difficile infection
Interactions
Monitoring
Clinical response by 48 to 72 hours
Failure at that point means a resistant organism, an abscess, or the wrong diagnosis
Diarrhoea
C. difficile
Counselling points
- Space the doses evenly through the day.
- Finish the course as agreed.
- Report severe or bloody diarrhoea rather than treating it yourself.
Clinical pearls
- It does not cover MRSA or enterococci. In a cellulitis that is not settling, that gap is usually the reason.
- It is a reasonable choice in most patients with a non-severe penicillin allergy, because side-chain similarity rather than the beta-lactam ring drives cross-reactivity.
References & review
- Cephalexin product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.