Cefalexin (Keflex): uses, side effects and cautions

Cefalexin belongs to the cephalosporins, a group related to the penicillins. The NHS lists pneumonia, chest infections, skin infections and urinary tract infections among its uses and notes that cefalexin is only available as capsules, tablets or a liquid; other cephalosporins can be given in other ways.

The NHS describes it as working by killing the bacteria behind the infection. It lists the brand name Keflex, and it is available only on prescription.

For most infections the NHS gives 500 mg two or three times a day, with more for severe infections and less for children. Swallow tablets and capsules whole with water, without chewing or breaking them. Liquid is measured with the supplied syringe or spoon.

Food does not matter. Space doses evenly, leaving at least 4 hours between them. Course length depends on the infection, with some needing at least 10 days; keep taking it until you finish, even if you feel better.

• Missed dose: take it when you remember unless the next is imminent. Never double up.

• One extra dose is unlikely to cause harm. Speak to a pharmacist or doctor if you took two or more extra doses, or you are worried.

• If there is no improvement after 2 to 3 days, or things worsen, tell your doctor.

Most adults and children can take cefalexin. The NHS asks you to mention an allergic reaction to it or other medicines, kidney problems, severe skin reactions to earlier antibiotics (rash, peeling, blistering or mouth sores) and severe or bloody diarrhoea after previous antibiotics.

The NHS page does not list penicillin allergy specifically, but it asks about allergy to other medicines, so make sure the prescriber knows about any reaction to a penicillin.

Up to 1 in 10 people get nausea, diarrhoea or stomach pain. Simple meals, taking the dose with food, plenty of fluids and, for cramps, resting and a warm compress help. Watch for dehydration, shown by passing less urine or dark urine.

• Call NHS 111 or a doctor for severe diarrhoea with blood or mucus, pale stools with dark urine or yellow skin or eyes, or unexplained bruising.

• Call 999 for facial or throat swelling, severe breathing difficulty, blue, grey or pale skin, confusion or collapse, or a swollen, raised, itchy, blistered or peeling rash.

The distinctive instruction is about zinc: do not take cefalexin at the same time as zinc supplements or anything containing zinc, and leave a gap of at least 3 hours before and after.

The NHS also names probenecid, metformin, diuretics such as furosemide and other antibiotics as medicines to review. Herbal remedies lack testing alongside it.

The NHS says cefalexin is safe at any stage of pregnancy, with no evidence of harm to the baby, and that treating infection in pregnancy matters. Only tiny amounts reach breast milk; diarrhoea has occurred in some babies, so watch for feeding changes, digestive upset, thrush or rash.

Alcohol is safe with it, there are no food restrictions and it does not affect driving or cycling. It does not weaken contraception apart from the effect of severe diarrhoea beyond 24 hours. The NHS explains antibiotic resistance as bacteria adapting and becoming less responsive, which is a reason not to skip doses.