A broad first-generation cephalosporin — useful, and usually a second choice rather than a first.
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Cefalexin is a first-generation cephalosporin antibiotic, closely related to penicillins and working the same way — by preventing bacteria from building their cell walls.
It covers a broader range of organisms than flucloxacillin or penicillin V, which is both its usefulness and the reason it is not a first choice. Broader cover means more collateral effect on the body's own bacteria, and cephalosporins carry a higher association with Clostridioides difficile than narrow-spectrum penicillins.
Where it earns its place is urinary infection in pregnancy, skin infection where a narrower drug is unsuitable, and situations where a mild penicillin allergy makes a penicillin inadvisable but a cephalosporin is still reasonable.
It does nothing for viral illness, and it is not the first-line choice for uncomplicated UTI in non-pregnant women — nitrofurantoin or trimethoprim usually are.
The liquid form needs refrigerating and is usually discarded after ten days.
The old teaching that ten percent of penicillin-allergic people react to cephalosporins is outdated. The genuine cross-reactivity rate with first-generation cephalosporins is low — low single figures at most, and lower still with modern agents.
What matters is the nature of the original reaction. A mild childhood rash is a very different proposition from throat swelling. A rash-type penicillin allergy does not usually preclude cefalexin; an anaphylactic one does.
Dose reduction is needed in kidney impairment. Considered acceptable in pregnancy and breastfeeding.
No routine monitoring for short courses. Kidney function matters for dosing in anyone with known impairment or in older people.
In pregnancy, a repeat urine sample after treatment is standard, because asymptomatic bacteriuria needs to be confirmed cleared rather than assumed.
Antibiotics do not reduce the effectiveness of the contraceptive pill — only enzyme-inducing drugs do, and cefalexin is not one. Vomiting or diarrhoea is the situation that warrants extra precautions.
Yes, where it is the appropriate choice — and part of the consultation is establishing whether it is. Cefalexin is frequently requested by name because someone has had it before, and a narrower antibiotic is often the better answer.
Where it is genuinely indicated: UTI in pregnancy, urinary infection where sensitivities point to it, skin infection in someone with a non-severe penicillin allergy, or where first-line options have failed.
A urine sample matters more than people expect. For anything recurrent, or in pregnancy, sending a sample before starting means the treatment can be corrected if the organism turns out to be resistant — and resistance rates make that a realistic scenario rather than a theoretical one.
What needs urgent in-person care: fever with loin pain or vomiting, suggesting kidney infection; a spreading skin infection in someone systemically unwell; any suspicion of sepsis — confusion, a very fast heart rate, breathlessness, or passing little urine. Those need assessment today, not a prescription.
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Cefalexin is an inexpensive generic. A standard course on a private prescription costs broadly what the England NHS prescription charge would. NHS prescriptions are free in Wales.
Fluid intake, paracetamol, and — for very mild symptoms in an otherwise well woman — waiting. A proportion of uncomplicated UTIs settle without antibiotics, and delayed prescribing is a recognised approach.
Cranberry products and alkalinising sachets have weak evidence and are not a substitute for treatment where treatment is needed.
Complete the course. Stopping early in urinary infection is a common cause of rapid recurrence.
It depends entirely on what the reaction was.
A mild rash years ago — usually yes. Cross-reactivity with first-generation cephalosporins is low, and the old ten percent figure is outdated.
Anaphylaxis, throat swelling or collapse — no. That rules out cephalosporins as well.
Describe the actual reaction rather than saying “I am allergic” — what happened, how quickly, and how long ago. That detail changes the answer.
Yes — it is one of the antibiotics considered acceptable in pregnancy, and it is commonly used for urine infections then. A follow-up urine sample afterwards is standard.
Usually because of pregnancy, kidney function, a resistant organism, or an allergy. Nitrofurantoin is first-line for uncomplicated UTI in most non-pregnant women.
Yes. Alcohol does not interact with cefalexin and does not stop it working. It will not help you feel better.
Urinary symptoms usually improve within 24 to 48 hours; skin infection within 48 to 72. No improvement by 72 hours is worth reporting rather than finishing the course and hoping.
Mild diarrhoea is common and usually settles. But severe, watery or bloody diarrhoea needs prompt advice — cephalosporins carry a higher C. difficile risk, and anti-diarrhoeal medicines are the wrong response.
No. Ordinary antibiotics do not affect the pill. Vomiting or diarrhoea does.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
September 5, 2026
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