Cephalexin

Key facts

• Cephalexin (a well-known brand is Keflex) is a first-generation cephalosporin antibiotic that kills susceptible bacteria by weakening their cell walls.
• You take it by mouth every 6 to 12 hours depending on the infection, with or without food, and you should finish the full course even after you feel better.
• Avoid cephalexin if you have had a severe, anaphylactic reaction to penicillin, cross-reaction between penicillins and cephalosporins is uncommon but documented and can be serious.
• Seek urgent care for hives, facial or throat swelling, breathing difficulty, or watery diarrhoea that will not stop.

What cephalexin treats

Cephalexin treats bacterial infections of the skin and soft tissue, streptococcal throat infections, urinary tract infections, middle ear infections, and bone infections such as osteomyelitis. It only works against bacteria, not viruses, so it will not help a cold or flu.

How cephalexin works

Bacteria build a rigid cell wall that keeps them intact under pressure. Cephalexin attaches to proteins the bacteria use to assemble that wall, blocking its construction. Without a working wall the bacterial cell takes on water, ruptures and dies, while your own cells, which do not have a cell wall, are unaffected.

Before you take it

• Do not take cephalexin if you have had a severe allergic reaction to any penicillin or cephalosporin antibiotic.
• Tell your prescriber if you have kidney disease, your dose may need to be lowered because cephalexin is cleared through the kidneys.
• Antacids containing aluminium or magnesium can lower how much cephalexin your body absorbs if taken at the same time; space them apart.
• Cephalexin can reduce the reliability of some oral contraceptives, use a backup method during treatment if this applies to you.

Side effects

Common effects include nausea, diarrhoea, headache, a mild skin rash, and vaginal itching or yeast infection.

Stop and seek urgent medical care for any of these:

• Hives, facial or throat swelling, or difficulty breathing.
• Watery diarrhoea that is severe or does not stop, which can signal a serious C. difficile gut infection.
• Yellowing of the skin or eyes, or a painful, spreading skin rash.

Safety essentials

• If you have ever had a severe reaction to penicillin, tell every prescriber and pharmacist before starting cephalexin, the antibiotic families are chemically related.
• Always finish the prescribed course. Stopping early lets surviving bacteria cause a relapse or grow resistant to treatment.
• People with reduced kidney function need a lower dose or longer interval between doses; make sure your prescriber knows your kidney status.
• Persistent or bloody diarrhoea during or after treatment needs prompt medical review, do not simply take an anti-diarrhoeal medicine and wait it out.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.