Cefalexin !

A first-generation oral cephalosporin for skin infections, pharyngitis, otitis and urinary infections due to staphylococci and streptococci.

Updated 2026-11-01

Class: Beta-lactams
Enzyme: Penicillin-binding proteins (bacterial transpeptidases)
ATC code: J01DB01
WHO Model List of Essential Medicines

Also called: cephalexin

Penicillin-binding proteins (bacterial transpeptidases)Cefalexin

The figure shows cephalothin, a first-generation cephalosporin with the same core, acylating the Streptomyces R61 DD-peptidase, the classic PBP model; no structure of this drug bound to a PBP has been published.

Target in 3D: Penicillin-binding proteins (bacterial transpeptidases). PDB structure 1CEG (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Like every cephalosporin, cefalexin has a β-lactam ring fused to a dihydrothiazine ring. It acylates the PBPs and stops peptidoglycan cross-linking. It is first generation: very active against gram-positive cocci (streptococci and methicillin-susceptible Staphylococcus aureus) and some enteric bacteria such as E. coli, Proteus mirabilis and Klebsiella. It does not cover enterococci, MRSA or Pseudomonas.

Absorbed from the gut and reaches the tissues
Acylates the PBPs of staphylococci and streptococci
Wall cross-linking stops
The bacterium lyses

Pharmacokinetics

Absorption: Good oral absorption.
Elimination: By glomerular filtration and tubular secretion; nearly all the dose leaves unchanged in the urine within a few hours.

Indications

Skin and soft-tissue infections (cellulitis, impetigo) due to staphylococci or streptococci. Cellulitis of the leg →
Streptococcal pharyngitis and respiratory infections by susceptible strains. Streptococcal pharyngitis →
Otitis media. Acute otitis media →
Genitourinary infections by susceptible strains. Urinary tract infection (acute cystitis) →
Bone infections due to S. aureus or P. mirabilis. Osteomyelitis →

Adverse effects

Common

Diarrhoea, nausea, dyspepsia
Rash, hives
Genital itching and vaginitis

Contraindications

History of hypersensitivity to this or another cephalosporin; caution, and sometimes contraindication, after anaphylaxis to penicillins or other β-lactams

Key interactions

Metformin: Cefalexin raises metformin concentrations; glucose and tolerance are monitored.
Probenecid: Reduces tubular secretion: higher and longer-lasting β-lactam levels.
Copper-reduction urine glucose tests: May give a false positive; enzymatic methods are used.

What is monitored

Clinical response
Kidney function in older adults or with kidney failure

Administration and safety

Oral, with or without food.
Do not confuse cefalexin with cefazolin or other cephalosporins with similar names.
Shake the reconstituted oral suspension before each use and store it as the pack says.

Nursing care

Before the first dose: ask about allergy to penicillins, cephalosporins and other β-lactams, and what the reaction was.
After the first dose, watch for hives, wheeze, low blood pressure or swelling of lips and tongue, with anaphylaxis equipment at hand.
Teach the patient to report watery or bloody diarrhoea, even weeks after the course ends.
Stress completing the course even if the skin or throat improves.

Pregnancy and breastfeeding

Available data do not show a higher risk of birth defects; it is used in pregnancy when the infection requires it.

Test yourself

Think of the answer, then tap the question to see it.

Which bacteria does no first-generation cephalosporin cover?

Enterococci, methicillin-resistant Staphylococcus aureus (MRSA), Listeria and Pseudomonas aeruginosa.

Which interaction with an oral antidiabetic does cefalexin's label describe?

It raises metformin concentrations, through competition for renal secretion.

How is cefalexin eliminated, and what does that mean in kidney failure?

Almost all of it leaves unchanged in the urine; in kidney failure it accumulates and the risk of adverse effects, including seizures, rises.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Cephalexin: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - CEPHALEXIN capsule (DailyMed (U.S. National Library of Medicine), 2026); RCSB PDB - 1CEG: CEPHALOTHIN COMPLEXED WITH DD-PEPTIDASE (RCSB Protein Data Bank, 2026); Farmacología básica y clínica (Katzung), cap. 43 (McGraw Hill, 2025).

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Representative model, for educational purposes only.