Key takeaways

Cephalexin is an FDA-approved antibiotic used to treat a variety of bacterial infections, working by preventing bacteria from making cell walls, which leads to their death.
Common side effects of cephalexin include nausea, vomiting, diarrhea, rash, headache, and dizziness, while serious side effects can include anaphylaxis, severe skin reactions, and Clostridium difficile-associated diarrhea.
Cephalexin should not be used by people who are allergic to cephalosporins. People who are allergic to penicillin may or may not be allergic to cephalexin, and should ask their healthcare provider for medical advice.
To avoid cephalexin side effects, patients should discuss their medical history with their doctor, review potential drug interactions, follow the prescribed dosage instructions, store the medication properly, and communicate any side effects to their healthcare provider.

Cephalexin (brand name Keflex) is a cephalosporin antibiotic, or antimicrobial, that is used to treat bacterial infections. Cephalexin treats bacterial infections such as respiratory tract infections, otitis media (middle ear infection), strep throat, skin infections, skin structure infections, bone infections, and urinary tract infections.

Cephalexin works by preventing bacteria from making cell walls, which causes the bacteria to die. Cephalexin is similar to penicillin. Cephalexin is approved by the FDA (United States Food and Drug Administration) and is available as a prescription drug.

More specifically, cephalexin works against infectious diseases caused by certain gram-positive bacteria, including:

Staphylococcus aureus (methicillin-susceptible)
Streptococcus pneumoniae (penicillin-susceptible)
Streptococcus pyogenes

And certain gram-negative bacteria, including:

Escherichia coli (E. coli)
Haemophilus influenzae
Klebsiella pneumoniae
Moraxella catarrhalis
Proteus mirabilis

Cephalexin is typically taken as a capsule by mouth two, three, or four times daily with or without food. It is also available as a tablet and oral suspension.

Cephalexin, like other antibiotics, can only treat bacterial infections and is not effective against viral infections such as the common cold, flu, or COVID-19. Using an antibiotic like cephalexin for a viral infection will not help treat the viral infection, and it can increase the development of drug-resistant bacteria.

Continue reading below to learn all about cephalexin and its side effects, warnings, and drug interactions.

Common side effects of cephalexin

Cephalexin, as with any drug, can cause side effects. The most common side effects of cephalexin are:

Nausea
Vomiting
Diarrhea
Rash
Headache
Dizziness
Increased liver enzymes
Eosinophilia (high level of a certain type of white blood cell)

Other, less common side effects of cephalexin may include:

Genital and anal itching
Genital fungal infection
Vaginal discharge
Fatigue
Joint pain

Serious side effects of cephalexin

Serious side effects, while less common, may occur. Serious possible side effects of cephalexin may include:

Anaphylaxis (serious allergic reaction)
Angioedema (swelling of the face and lips)
Erythema multiforme (bullseye-shaped lesions on the skin or mouth)
Stevens-Johnson syndrome (a medical emergency characterized by flulike symptoms, rash, and blisters)
Toxic epidermal necrolysis (a life-threatening skin disorder with symptoms of skin peeling and blistering)
Clostridium difficile-associated diarrhea (also called CDAD; symptoms can range from mild diarrhea to life-threatening colitis)
Neutropenia (low neutrophil count, a type of white blood cells)
Thrombocytopenia (low platelet count)
Hemolytic anemia (destruction of red blood cells)
Aplastic anemia (the body stops making enough new blood cells)
Hemorrhage (bleeding)
Hepatitis (inflammation of the liver)
Cholestatic jaundice (bile flow from the liver slows or stops; symptoms include itching, dark urine, and yellowing of the skin or eyes)
Seizures (especially in people with kidney problems who did not take a reduced dose of cephalexin)

Cephalexin and hypersensitivity reactions

Cephalexin is contraindicated in people who are allergic to cephalexin or any drug in the cephalosporin class of drugs. Before taking cephalexin, discuss any allergies to medication with your healthcare provider. Up to 10% of people with a penicillin allergy may be allergic to cephalexin.

Allergic reactions may manifest in the form of a skin reaction (rash, hives, itching, blisters, peeling), angioedema, or trouble breathing. If you experience any of these symptoms, stop taking cephalexin and seek emergency medical treatment.

Cephalexin and Clostridium difficile-associated diarrhea (CDAD)

Any antibiotic, including cephalexin, can cause CDAD. Severity may range from mild diarrhea to fatal colitis. CDAD can occur during treatment and up to two months after antibiotic treatment. If you are taking cephalexin and you have diarrhea, or you experience diarrhea up to two months after finishing cephalexin, contact your doctor. If symptoms are severe, seek emergency medical treatment.

How long do cephalexin side effects last?

Cephalexin side effects tend to be mild and go away as your body adjusts to the medication or when you finish taking the medication. If you are taking cephalexin, it is important to take it for the full course prescribed by your healthcare professional. Cephalexin does not tend to cause long-lasting side effects. However, some patients get CDAD, which can occur during cephalexin treatment or up to two months after treatment. If you have diarrhea after you finish cephalexin, be sure to tell your doctor that you took cephalexin.

If you have bothersome or severe adverse effects, call your doctor or seek emergency treatment as required.

Cephalexin contraindications & warnings

Abuse and dependence

Cephalexin is not a controlled substance. It is not habit-forming. When the course of treatment is complete, you can stop taking cephalexin. There is no need to taper off the medication.

Overdose

The maximum total daily dose of cephalexin is 4000 mg per 24 hours, divided every six hours or every 12 hours. A standard dose of cephalexin is 500 mg every six hours for a total daily dose of 2000 mg. Older adults and people with impaired kidney function generally take a lower dose of cephalexin.

Overdose symptoms may include nausea, vomiting, stomach pain or rib pain, diarrhea, and blood in the urine. Contact your doctor or the poison control center for information on cephalexin overdose.

Restrictions

People who are hypersensitive to cephalexin or cephalosporins should not take cephalexin. People who are allergic to penicillin may be allergic to cephalexin, too, and should consult a healthcare provider before using cephalexin.

Cephalexin should be used with caution in people who have:

Kidney disease
Recent antibiotic-associated colitis (CDAD)
A history of gastrointestinal problems

Cephalexin is classified as a pregnancy category B drug. Because there is not enough data in pregnant women, cephalexin should only be used during pregnancy if clearly needed and under close supervision of the healthcare professional. Cephalexin is excreted in breast milk. If you are pregnant or breastfeeding, consult your doctor for medical advice.

Cephalexin is safe in children at recommended dosages.

People with kidney problems are generally prescribed a lower dose of cephalexin, depending on the creatinine clearance.

In older adults, cephalexin is safe, but this population is more likely to have decreased kidney function, so dosing may be lower.

Cephalexin interactions

Metformin: Cephalexin increases metformin levels, which can cause hypoglycemia (low blood glucose levels) or other side effects of metformin. If the medications must be taken together, carefully monitor blood sugar. A dose adjustment of metformin may be required while taking cephalexin.
Dofetilide: Cephalexin and dofetilide should not be used together. The combination can cause increased dofetilide levels, leading to QT prolongation and arrhythmias (irregular heartbeat).
Probenecid: Cephalexin and probenecid should not be used together. This combination can increase levels of cephalexin and cause increased side effects of cephalexin.

Although these are the most common drug interactions, other drug interactions may occur. Consult your healthcare provider for a full list of drug interactions.

How to avoid cephalexin side effects

Medical history: Discuss medical conditions and your medical history with your doctor, so he or she can determine if cephalexin is safe for you.
Interactions: Because cephalexin has potential drug interactions, be sure to review all of the medications you take with your prescriber. This includes prescription drugs, over-the-counter medicines, and any vitamins or dietary supplements you take.
Dosage: Follow the instructions given by your doctor. Take cephalexin for the full course of treatment. Do not stop taking it early, even if you are feeling better unless your doctor instructs you to do so. Stopping the medication early may result in a regrowth of bacteria. Your doctor will instruct you on the proper length of time to take cephalexin.
Storage: Store your medication properly. Store cephalexin capsules at room temperature, away from heat, light, and moisture. Cephalexin oral suspension should be stored in the refrigerator for up to 14 days. Discard after the course of treatment is complete.
Communicate: Talk to your doctor and/or pharmacist. If you experience mild side effects, your healthcare provider may be able to give you tips on avoiding them. For example, cephalexin can be taken with or without food, but if it causes stomach issues, you may want to try taking it with food. Keep a journal where you can jot down notes about any side effects and what helps them. If you have serious side effects, tell your doctor right away or seek emergency medical attention when needed.
Read the patient leaflet: When you fill your cephalexin prescription, you will receive a patient information leaflet with drug information about side effects and warnings. Read the information, and ask your pharmacist if you have any questions.