Cephalexin (Keflex): Uses, Side Effects, Dosages, Contra-indications, Warnings, Interactions & More

By Zina Semenovskaya, MD
Medically reviewed
July 2, 2021

When you have a common bacterial infection such as a middle ear infection, respiratory tract infection, or urinary tract infection, there’s a good chance your doctor will prescribe a drug called cephalexin.

Cephalexin monohydrate (Keflex) is a prescription drug used to kill the bacteria causing these infections. It is similar to penicillin and is among the top 100 most commonly prescribed drugs in the United States.

In this article, I’ll explain all about cephalexin, including how it works, common conditions it’s used to treat, side effects, and if you can get cephalexin online. 

Chat with a doctor and get an antibiotic prescription for just $23

Get started

What Is Cephalexin?

Cephalexin is a type of antibiotic called a cephalosporin. It works to prevent the growth of the bacterial cell wall. This keeps bacteria from multiplying and helps eradicate an infection.

Cephalexin is considered a broad-spectrum antibiotic because it’s effective in fighting many common bacteria, including:

  • Streptococcus pneumoniae 
  • Streptococcus pyogenes
  • Haemophilus influenzae
  • Staphylococcus aureus
  • Moraxella catarrhalis 
  • Escherichia coli 
  • Proteus mirabilis
  • Klebsiella pneumoniae

What is it used for?

Cephalexin is prescribed to treat infections caused by bacteria, including: 

  • Respiratory tract infections
  • Sinusitis
  • Otitis media (middle ear infections)
  • Skin and soft tissue infections
  • Genitourinary infections (urinary tract infections)
  • Endocarditis prevention

Can You Get Cephalexin Online?

In order to get cephalexin, you first need a prescription. You can get this online through your healthcare provider.

Once you receive the prescription, you can purchase cephalexin online. Before buying any medicine on the web, though, consider these tips from the Food and Drug Administration (FDA):

  • Only use sites that require a prescription and have a pharmacist available for questions.
  • Only buy prescription drugs from licensed US pharmacies.
  • Don’t share your credit card information or other personal information unless the site is secure.

What Forms Does Cephalexin Come In?

Cephalexin is available as a liquid, capsule, or tablet taken by mouth. If you take the liquid form, store it in the refrigerator and shake it well before using.

Do not store the tablets or capsules in the bathroom. Instead, keep them in a dry place at room temp. When it’s time for a dose, swallow the pill whole with a glass of water. 

What Are Common Doses of Cephalexin?

Doctors prescribe different doses of cephalexin depending on the patient’s age, infection, and severity of their condition. Below are some of the most common doses of cephalexin for different infections.

Upper respiratory tract infections (including tonsillitis)

  • Adults: 1-4 grams (g) daily, divided in 2-4 doses (generally 250 milligrams (mg) taken orally every 6 hours or 500 mg taken orally every 12 hours) for 7-14 days.
  • Children and adolescents: 25-50 mg per kilogram of body weight per day taken orally in 2-4 divided doses for 7-14 days.

Lower respiratory tract infections (LRTIs)

  • Adults: 250 mg taken orally every 6 hours or 500 mg taken orally every 12 hours for 7-14 days. 
  • Children and adolescents: 25-50 mg per kilogram of body weight per day taken orally divided in 2-4 doses for mild to moderate infections. For severe infections, 50-100 mg per kilogram of body weight per day taken orally divided in 3-4 doses.

Skin and skin structure infections (including impetigo) 

  • Adults: 1-4 g daily, divided in 2-4 equal doses (generally 250 mg take orally every 6 hours or 500 mg taken orally every 12 hours) for 7-14 days. 
  • Children and adolescents: 25-50 mg per kilogram of body weight per day taken orally in 2-4 divided doses for 7-14 days.

Otitis media (middle ear infection)

  • Children and adolescents: 75-100 mg per kilogram of body weight per day taken orally in 3-4 divided doses. 

Bone and joint infections

  • Adults: 1-4 g daily, divided in 2-4 doses (generally 250 mg taken orally every 6 hours or 500 mg taken orally every 12 hours) for 7-14 days. 
  • Children and adolescents: 75-100 mg per kilogram of body weight per day taken orally in 3-4 divided doses. 

Genitourinary infection (including urinary tract infection)

  • Adults: 1-4 g daily, divided in 2-4 doses (generally 250 mg taken orally every 6 hours or 500 mg PO every 12 hours) for 7-14 days.
  • Children and adolescents: 25-50 mg per kilogram of body weight per day taken orally in 2-4 divided doses for 7-14 days.

Side Effects

As with all medications, cephalexin can cause adverse effects, with diarrhea being most common. Other rare, but not uncommon, side effects include:

  • Abdominal pain
  • Indigestion
  • Vomiting
  • Nausea
  • Headache
  • Fatigue
  • Vaginal discharge or itchiness
  • Increased creatinine 

Cephalexin and other cephalosporin antibiotics such as cefaclor, cefadroxil, and cefazolin can also cause a severe allergic reaction known as anaphylaxis.

Symptoms include hives, trouble breathing, and swelling of the face, lips, tongue, or throat.

Call your doctor right away if you think you are experiencing an allergic reaction or other more serious side effects after taking cephalexin. 

How to Take Cephalexin

Cephalexin comes as an oral capsule, tablet, or liquid that can be taken with or without food. Depending on the condition being treated, adults usually take it every six or 12 hours for 7-14 days.

Either way, take cephalexin around the same times every day.

If you miss a dose of cephalexin, take that missed dose as soon as possible, unless you are close to your next dosing time. In this case, skip the missed dose and wait until the next one. Never double up doses.

Contraindications

Be sure your healthcare provider knows if you have any of the below conditions or sensitivities, as cephalexin can lead to serious complications in these cases:

  • Colitis
  • Kidney disease
  • Liver disease
  • Seizure disorders
  • Hypersensitivity to penicillin

Breastfeeding or pregnant women should also consult with a physician before using cephalexin, as should individuals undergoing dialysis.

Chat with a doctor and get an antibiotic prescription for just $23

Get started

Drug interactions

Cephalexin can negatively interact with certain medications, increasing the risk of side effects.

Ask your doctor whether it would be safe for you to use cephalexin if you take any of the following:

  • Metformin
  • Furosemide
  • Warfarin or other blood thinners
  • Probenecid

Warnings

Specific groups of people should be especially sure to consult their doctor before taking cephalexin.

  • People with kidney problems: If you have kidney problems or a history of kidney disease, you may not be able to clear this drug from your body. This may increase the levels of cephalexin in your body and cause more side effects. 
  • Pregnant women: Cephalexin is a pregnancy category B drug. That means animal studies have shown no risk to a fetus, but there’s insufficient evidence to show if the drug poses a risk to human fetuses. 
  • Breastfeeding women: Cephalexin is passed through breast milk. Talk to your doctor about the potential risks for your infant.
  • Seniors: The risk of kidney problems increases as you age. Reduced kidney function can cause drugs to stay in the body for a longer time, increasing the chance of side effects.
  • Those who are allergic to beta-lactam medications: Do not take cephalexin. You could have a serious allergic reaction.

In case of emergency or overdose

In case of an overdose, call the poison control center at 1-800-222-1222. Information is also available online. If you collapse, have a seizure, have trouble breathing, or cannot be awakened, have someone immediately call 911.

Get a Cephalexin Prescription Today with K Health

K Health provides a simple, accessible option for prescription treatment. Chat with a doctor on your phone to determine whether you need a prescription. Your doctor will then prescribe you medication, which can be picked up at a local pharmacy or shipped discreetly directly to you.

Frequently Asked Questions

What is cephalexin used for?
Cephalexin is used to treat bacterial infections in various parts of the body. This includes respiratory tract infections, ear infections, skin infections, and urinary tract infections. It works by killing or preventing the growth of bacteria.
How long does cephalexin stay in your system?
You’ll pee out more than 90% of cephalexin within eight hours of taking it. However, if you have reduced kidney function, this process can take longer.
What are the side effects of cephalexin?
The more common side effects of cephalexin oral capsules include diarrhea, indigestion, nausea, vomiting, or stomach pain. Most of these side effects should go away on their own. If they persist, contact your doctor.
How quickly does cephalexin work to treat an infection?
Cephalexin starts to work right away and becomes the most effective one hour after taking it. That being said, it can take up to 48 hours after taking the drug to start seeing your symptoms improve.
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Zina Semenovskaya, MD

Dr. Semenovskaya specializes in emergency medicine, and received her medical degree from Weill Cornell Medical College. She is currently the medical director at Remote Emergency Medicine Consulting, LLC and splits her time working clinically as an emergency medicine attending in California and Alaska. She is the first of our doctors to be fluent in Russian.