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What is Keflex (cephalexin)?

Keflex is a prescription antibiotic medication that’s available as a tablet, capsule, and liquid suspension. The generic drug is cephalexin.

The medication is used to treat a variety of bacterial infections and belongs to a class of antibiotics called cephalosporins.

What is Keflex used to treat?

Keflex is most commonly used to treat bacterial infections of the skin, ear, bone, and urinary tract. It can also be used to treat certain types of pneumonia.

Keflex is a cephalosporin antibiotic. This class of antibiotics works by inhibiting the bacteria’s cell wall synthesis, which eventually causes it to rupture and die.

How do you take Keflex?

Keflex is available in capsule, tablet, and suspension (liquid) form for oral use.

It is often taken every six or 12 hours, with or without food, for seven to 14 days,¹ depending on the condition being treated.

Try to take Keflex at around the same time each day to reduce the risk of forgetting a dose. Take care to carefully follow the recommendations on your prescription label, and ask your doctor or pharmacist to explain any section you do not understand.

Keflex should be taken exactly as instructed. Do not take a lower or higher dose of Keflex than your doctor prescribed, or take it more often.

Seeing results

Keflex starts to take effect straight away, but you may need to wait for 48 hours² before you notice any improvements.

Keflex is usually taken for seven to 14 days.³ If your infection is severe, you may need to take it for a longer period of time. Let your doctor know if your symptoms do not improve or get worse.

Potential side effects of Keflex

Like all medications, Keflex can cause side effects.

The most common side effects of Keflex include:

While these side effects are typically mild and go away on their own, some people experience more severe side effects. If you experience any of the following side effects, stop taking Keflex and call your doctor immediately:

  • Hives

  • Difficulty breathing

  • Swelling of the face, lips, tongue, or throat

  • Yellowing of the skin or eyes

  • Dark urine

  • Fever

  • Rash

Clostridium difficile

Like other antibiotics, Keflex treatment may cause complications caused by Clostridium difficile (C. diff) bacteria. C. difficile-associated disease (CDAD) can occur during the months following cephalexin therapy.

Common complications of C. diff include dehydration, severe diarrhea, and colitis, which may be fatal in rare cases.

Long-term use of Keflex

Talk to your doctor about the benefits and risks of taking Keflex long-term.

Taking antibiotics for longer than two months in late adulthood has been found to increase the risk of death in women due to cardiovascular diseases.⁴

Long-term Keflex use may also lead to the development of drug-resistant bacteria, oral thrush, and yeast infections.

Missed doses

If you miss a dose of Keflex, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not take more than what is prescribed.


Symptoms of a Keflex overdose may include:

  • Drowsiness

  • Agitation

  • Seizures

  • Blood in urine

  • Nausea and vomiting

Take Keflex exactly as prescribed by your doctor; don’t take more or less of it or take it more often than recommended.

What to discuss with your doctor before taking Keflex

It’s important to tell your doctor if you have ever had an allergic reaction to Keflex or another antibiotic.

Your doctor will also want to know if you have a kidney condition,⁵ as you might need to take a lower dosage to avoid complications.

Be sure to tell your doctor about any other medications you are taking, as Keflex may interact with them.

You should also let your doctor know if you are pregnant or breastfeeding, as Keflex may not be a suitable medication for you.

Stopping Keflex

You should only stop taking Keflex when your prescription ends or your doctor tells you to.

Your infection may not be completely treated and the bacteria may become resistant to antibiotics if you stop taking Keflex too soon. Regularly skipping doses may have the same effect.

Keflex and pregnancy

Keflex is listed under the US Food and Drug Administration’s (FDA) pregnancy category B,⁶ meaning the medication is not expected to harm an unborn baby. Your doctor may prescribe Keflex during pregnancy, but they will discuss any risks with you beforehand.

Keflex passes into breast milk, so you must make your doctor aware if you are breastfeeding before taking this medication.

Tell your doctor immediately if you find out you are pregnant while taking Keflex.

Interactions with other drugs

Keflex can interact with other medications, so give your doctor a complete list before taking this drug. Don’t start taking anything new while undergoing Keflex treatment without speaking to your doctor first. This includes over-the-counter medications, vitamins, and herbal products.

An interaction is when a substance changes the way a drug works. This can cause complications, like increased side effects, or it may prevent the drug from working effectively.

Keflex can interact with:

  • Anticoagulants (blood thinners) such as warfarin (Coumadin)

  • Probenecid (Benemid)

  • Diuretics (including furosemide)

  • Live vaccines

  • Other antibiotics

  • Metformin

Dosage adjustments, tests, and monitoring may be necessary for you to take Keflex safely if you are also taking any of these drugs.

Allergy information

Cephalexin can cause a life-threatening allergic response. Symptoms may include:

  • Hives

  • Breathing difficulties

  • Swollen face, lips, tongue, or throat

If you experience an allergic response, seek emergency medical help immediately.

Do not use this medication again if you have previously experienced an allergic response to it. Retaking it may cause fatal complications.

Tips and advice for taking Keflex

Here are some tips to help you take Keflex safely and effectively:

  • Take Keflex at evenly spaced intervals throughout the day. This will help keep a consistent level of the drug in your system.

  • Don’t skip doses of Keflex. If you forget to take a dose, take it as soon as you remember. If it’s almost time for your next dose, skip the missed dose and continue on your regular schedule.

  • Don’t stop taking Keflex early, even if you feel better. Finish the full course of treatment to make sure the infection is completely gone.

If you have any questions or concerns about taking Keflex, talk to your doctor. They can help create a treatment plan that’s right for you.

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.