Cephalexin
Cephalexin is a first-generation oral cephalosporin antibiotic used for skin and soft tissue, respiratory, bone and urinary tract infections and otitis media. Originally launched as Keflex by Eli Lilly, it is now one of the most widely prescribed generic antibiotics worldwide.
- Chemical formula
- C16H17N3O4S
- CAS number
- 15686-71-2
- ATC code
- J01DB01
- Molecular weight
- 347.39 g/mol
- Drug class
- First-generation cephalosporin antibiotic
- Also known as
- Cefalexin, Keflex, Cefalexina, Cephalexinum
What is it?
Cephalexin (spelled cefalexin in the INN) is a semi-synthetic first-generation cephalosporin derived from 7-aminodesacetoxycephalosporanic acid, approved by the FDA in 1971 and launched as Keflex by Eli Lilly. It was the first cephalosporin that could be given by mouth with reliable absorption, which made the class practical outside hospitals. Generic cephalexin capsules, tablets and paediatric suspensions are available in almost every country, and the molecule is on the WHO List of Essential Medicines.
Mechanism of action
Cephalexin is a beta-lactam antibiotic. Its beta-lactam ring acylates the active site of penicillin-binding proteins (PBPs), the transpeptidases that cross-link peptidoglycan strands in the bacterial cell wall. Without cross-linking the wall cannot contain the cell's osmotic pressure, and growing bacteria lyse, so the effect is bactericidal and time-dependent. The spectrum covers streptococci, methicillin-susceptible staphylococci and selected Gram-negative rods (E. coli, Proteus mirabilis, Klebsiella pneumoniae). It is inactive against MRSA, enterococci, Pseudomonas and most anaerobes, and is hydrolysed by extended-spectrum beta-lactamases.
Pharmacokinetics
Cephalexin is acid-stable and almost completely absorbed after oral dosing, with bioavailability above 90%; food slows absorption slightly without reducing it. Peak plasma concentrations of about 18mg/L after a 500mg dose occur within one hour. Protein binding is low (10–15%) and the drug distributes into skin, bone, synovial fluid and pleural fluid but penetrates poorly into cerebrospinal fluid. The elimination half-life is roughly one hour in adults with normal renal function. Cephalexin is not metabolised; over 90% is excreted unchanged in urine by glomerular filtration and tubular secretion, so dose reduction is needed when creatinine clearance falls below 60mL/min.
Indications
Cephalexin is approved for respiratory tract infections, otitis media, skin and skin-structure infections, bone infections and genitourinary infections including acute prostatitis. In everyday practice it is used for uncomplicated cellulitis, impetigo, infected wounds, streptococcal pharyngitis and uncomplicated cystitis, and it is a common choice for urinary tract infections in pregnancy. Usual adult dosing is 250–500mg every 6 hours or 500mg every 12 hours, to a maximum of 4g per day; children receive 25–50mg/kg/day in divided doses. A doctor should confirm the diagnosis and choose the dose and duration.
Safety profile
Cephalexin is generally well tolerated. The most common adverse effects are gastrointestinal: diarrhoea, nausea, dyspepsia and abdominal pain. Hypersensitivity reactions range from urticaria and rash to rare anaphylaxis; cross-reactivity in patients with a confirmed penicillin allergy is now estimated at about 1 to 3%, with the highest risk in those who reacted to aminopenicillins sharing a similar side chain. As with all antibacterials, Clostridioides difficile-associated diarrhoea can occur during or up to two months after treatment. Rare reports include interstitial nephritis, cholestatic hepatitis, haemolytic anaemia and, at very high doses in renal impairment, seizures. Cephalexin is considered compatible with pregnancy and breastfeeding.
Products containing this ingredient
Frequently asked questions
How is cephalexin different from amoxicillin? ▾
Both are oral beta-lactams with a similar Gram-positive spectrum, but cephalexin is more stable to staphylococcal penicillinase, which makes it a better choice for skin infections where Staphylococcus aureus is likely. Amoxicillin covers more streptococcal respiratory pathogens, Helicobacter pylori and Listeria, and is generally preferred for ear, sinus and chest infections. Cephalexin has a shorter half-life and is usually taken more often. The choice depends on the likely organism and the patient's allergy history.
Is cephalexin safe for people with a penicillin allergy? ▾
Usually, but not always. Current evidence puts cross-reactivity between penicillins and cephalexin at about 1 to 3% in people with a confirmed penicillin allergy, far lower than the 10% figure once quoted. The risk is highest after a severe or immediate reaction, and in patients allergic to amoxicillin or ampicillin, whose side chains resemble cephalexin's. Anyone with a history of anaphylaxis to a penicillin should avoid cephalexin unless an allergist advises otherwise.
How long does cephalexin stay in the body? ▾
The half-life is about one hour in adults with normal kidney function, so the drug is essentially cleared within 6 to 8 hours of the last dose. This short half-life is why cephalexin is dosed every 6 to 12 hours. In renal impairment the half-life lengthens considerably, and the dose or interval must be adjusted by the prescribing doctor.
Can cephalexin be used for a urinary tract infection? ▾
Yes. Cephalexin is excreted unchanged in urine, reaching concentrations many times higher than in blood, and is active against E. coli, Proteus and Klebsiella, the usual causes of cystitis. It is a standard option for uncomplicated urinary tract infections and one of the preferred antibiotics in pregnancy. Local resistance rates vary, so a urine culture is advisable for recurrent or complicated infections.
What should I do if I develop diarrhoea on cephalexin? ▾
Mild loose stools are common and usually settle after the course. Severe, watery or bloody diarrhoea, especially with abdominal cramps or fever, may indicate Clostridioides difficile infection, which can appear during treatment or up to two months afterwards. Stop taking anti-diarrhoeal medicines, keep hydrated and contact a doctor promptly, as this requires specific treatment.
Sources
This page is compiled from the official prescribing information for Cephalexin. The links below open a search for that active ingredient in each register, so you always reach the current version of the label rather than a cached copy.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.