Cephalexin: doses, uses, and side effects

You’ve got an infection, and your healthcare provider has prescribed cephalexin. You’re probably wondering what to expect with this medication. Keep reading to learn about what cephalexin is used for, what dosages you might be prescribed, and possible side effects and warnings. 

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Cephalexin (brand name Keflex) is an antibiotic used to treat a range of common bacterial infections. Cephalexin destroys bacteria by preventing them from building a cell wall they use for support and protection. 

The FDA has approved cephalexin for the treatment of conditions caused by susceptible bacteria, including (FDA, 2005): 

• Skin infections

• Infections of the nose and throat, including “strep throat”

• Ear infections (otitis media)

• Pneumonia

• Bone infections

• Urinary tract infections (UTIs) and prostatitis

Typically, your healthcare provider will prescribe cephalexin for a short course of treatment, about 7–10 days. 

Cephalexin can also be used “off-label” to treat other conditions, meaning the FDA didn’t explicitly approve it for those uses. Healthcare providers can prescribe drugs for an unapproved use if they decide that it’s the best treatment for their patients.

Off-label uses for cephalexin include treating infections of prosthetic joints (like hip or knee replacements) and prevention of endocarditis, an infection of the heart that people with certain conditions may be at risk for after dental work or surgical procedures.

Occasionally, some infection-causing bacteria will be resistant to cephalexin. In those cases, your healthcare provider will prescribe an alternative antibiotic. 

Like all medications, cephalexin can potentially cause adverse effects. However, mild side effects tend to be more common, while severe side effects are usually rare.

Gastrointestinal symptoms

The most common side effects of cephalexin are stomach pain and diarrhea (FDA, 2005). This is because cephalexin, like all antibiotics, can’t tell the difference between infectious bacteria and the normal bacteria we have in our intestines and elsewhere. Those normal bacteria play a vital role in helping our bodies function well. When we take an antibiotic, some of the normal bacteria in our intestines are destroyed along with the infectious bacteria, leading to gastrointestinal complaints like diarrhea and discomfort. Diarrhea caused by antibiotics usually goes away on its own.

On very rare occasions, an antibiotic weakens our intestines’ normal bacteria enough that a harmful bacteria called Clostridium difficile (C. diff). C. diff can flourish, causing a condition known as “pseudomembranous colitis.” This is marked by prolonged, severe diarrhea and requires prompt treatment. Contact your healthcare provider if you are taking or have recently taken an antibiotic and you experience diarrhea that lasts more than a few days. 

Allergic Reactions

Some people may be allergic to cephalexin. Signs of an allergy typically include rash, itchiness, or swelling. Less commonly, swelling may occur in the airway, leading to trouble breathing and a process called anaphylaxis. Contact your healthcare provider if you are experiencing any signs of an allergic reaction.

Up to 10% of people with a penicillin allergy may also be allergic to cephalexin (FDA, 2005). 

Less common side effects

Less frequently, some people experience other side effects such as (FDA, 2005): 

• Confusion, dizziness, hallucination, headache, or fatigue

• Jaundice

• Blood cell disorders

• Arthritis

• Vaginal discharge

Cephalexin comes in doses of 250 mg, 500 mg, and 750 mg. It is also available in liquid form, often prescribed to young children. 

You’ll typically take cephalexin every 6 to 12 hours, with or without food. 

Your healthcare provider will determine the correct dosing based on your particular infection. Cephalexin dosing in young children goes based on their weight. A healthcare provider may adjust the dose based upon your symptoms, additional medical conditions, or any other medications you are taking. 

People who’ve had allergic reactions to penicillins or antibiotics in the same family as cephalexin (called cephalosporin antibiotics) should speak with their healthcare provider before taking cephalexin.

Cephalexin is a Pregnancy Category B medication, meaning that animal studies have not shown any risk to a fetus, and there aren't adequate studies in humans (HHS, 2021). Pregnancy Category B drugs are generally considered safe to give to pregnant women when indicated. Cephalexin is often given for both infection treatment and prevention during pregnancy.

Cephalexin does pass into breastmilk, but in many cases, breastfeeding women can still take it when needed (FDA, 2005). If you are nursing or pumping breastmilk, let your healthcare provider know before taking cephalexin. Your provider will weigh any risks and benefits to you or your child when deciding on the right antibiotic.

Antibiotics, like cephalexin, do not treat viral infections, including the common cold. In fact, the FDA has a black box warning, its highest level warning, discouraging the use of cephalexin for non-bacterial infections as doing so may encourage antibiotic resistance. 

Drug interactions change how your medications work and can potentially increase your risk for serious side effects. 

Cephalexin can interact with a few other medications, including (FDA, 2005):

• Metformin—Taking cephalexin and metformin, a drug commonly prescribed to people with diabetes to help control blood glucose, can increase the concentration of both medications in your body. This can potentially lead to a higher risk of developing side effects.

• Probenecid—This common gout medication can slow down how quickly your kidneys filter out cephalexin. Like the interaction between metformin and cephalexin, probenecid can increase cephalexin's concentration in your body and potentially increase your risk for side effects. 

As with all medications—especially with antibiotics—it's essential that you follow your healthcare provider's instructions and complete the full course you've been prescribed. Then, if you have any side effects or concerns, speak with your provider to determine the best approach for you.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

May 04, 2021

Written by

Steve Silvestro, MD

Fact checked by

Felix Gussone, MD

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About the medical reviewers

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.