Cephalexin, also known by its brand name Keflex, is an antibiotic used to treat various types of infections caused by bacteria. It belongs to a group of drugs called cephalosporin antibiotics. The usual adult dose is 250 mg or 500 mg, which is given 2 to 4 times per day. Larger doses are used for severe infections. Cephalexin is generally well tolerated and safe, but it may produce side effects. These can range from relatively common effects, such as mild diarrhea or stomach upset, to rare but severe side effects, such as certain types of allergic reactions.
Common Side Effects
Various gastrointestinal symptoms, including mild diarrhea, nausea, vomiting, stomach upset and abdominal pain, are among the most common side effects of cephalexin. Dizziness, fatigue and headaches are also relatively common. Some people experience itching in the genital region or around the anus when they take cephalexin. Genital yeast infections can develop in men or women, as cephalexin kills not only the disease-causing bacteria but also the good bacteria that normally live in and on the body. Death of the good bacteria allows microorganisms causing yeast infections to flourish.
Like any drug, cephalexin can cause allergic reactions. These may include hives, a mild rash, or angioedema -- swelling of the skin or the mucous membranes lining the inside of the mouth or throat. Rarely, a potentially life-threatening anaphylactic reaction may occur, with rapid development of generalized hives, wheezing, shortness of breath, lightheadedness or loss of consciousness. Stevens-Johnson syndrome, toxic epidermal necrolysis, and erythema multiforme are other rare allergic reactions, which produce widespread skin lesions. The most serious of the three, toxic epidermal necrolysis, is characterized by extensive peeling of the skin and mucous membranes that can be fatal.
Other Uncommon Side Effects
As with many antibiotics, cephalexin may occasionally cause a severe form of diarrhea called antibiotic-associated diarrhea or pseudomembranous colitis. This condition is usually due to overgrowth of a bacteria called Clostridium difficile in the intestines, which occurs because of destruction of the normal good bacteria in the gut. Liver inflammation and dysfunction are other possible, yet rare, side effects of cephalexin. They can produce abnormal liver blood tests and yellowish discoloration of the skin, called jaundice. Fortunately, these effects are usually temporary and resolve after the drug is stopped. A type of kidney inflammation called interstitial nephritis is another rare, transient side effect. On rare occasions, cephalexin leads to reduced numbers of any of the three main types of blood cells -- red blood cells, infection-fighting white blood cells, and clot-producing platelets.
Warning and Precautions
Cephalexin, like other antibiotics, should only be used for infections proven or strongly suspected of being due to bacteria. It should also be taken for the entire length of time it is prescribed, as early discontinuation may lead to worsening of the infection.
Discontinue cephalexin and obtain prompt medical care if you have any symptoms of an allergic reaction. Seek prompt medical attention if you have severe diarrhea, especially if it is accompanied by symptoms of dehydration such as reduced urination, weakness or lightleadedness. Before taking cephalexin, tell your doctor if you have any allergies. People who are allergic to any of the penicillin group of antibiotics may have an increased likelihood of developing an allergic reaction to cephalosporins. Also tell your doctor if you are pregnant or breastfeeding. Cephalexin may be safe for your unborn child or nursing infant, but this has not been conclusively proven.
Reviewed by: Mary D. Daley, M.D.