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Keflex: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Apr 8, 2020.

1. How it works

  • Keflex is a brand (trade) name for cephalexin. Cephalexin is an antibiotic that is used to treat infections caused by susceptible bacteria. Cephalexin is bactericidal (kills bacteria) and works in a similar way to penicillins. It binds to and blocks the activity of enzymes responsible for making peptidoglycan, an important component of the bacterial cell wall. Cephalexin is called a broad-spectrum antibiotic because it is effective against a wide range of bacteria.
  • Keflex belongs to the class of antibiotics known as cephalosporins. Keflex is a first-generation cephalosporin and is mainly effective against gram-positive bacteria.

2. Upsides

  • Keflex treats a wide range of infections such as those occurring in the respiratory tract, ear, genitourinary area, bone, or on the skin.
  • Keflex has excellent activity against gram-positive staphylococci and streptococci bacteria, including susceptible isolates of Staphylococcus aureus, Streptococcus pneumoniae, S. pyrogens, Haemophilus influenzae, Klebsiella pnumoniae, Moraxella catarrhalis, and Proteus mirabilis.
  • May be given twice daily.
  • May be given with or without food.
  • Keflex is available as a generic under the name cephalexin.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Diarrhea, dyspepsia, abdominal pain, and nausea, have been reported. May alter some laboratory tests. Seizures have been reported rarely.
  • Approximately 10% of people who are allergic to penicillin are also allergic to cephalosporins. Tell your doctor if you have ever had an allergy to antibiotics.
  • The dosage of Keflex requires adjusting for people with moderate-to-severe kidney disease.
  • Severe diarrhea, caused by overgrowth of a bacteria called Clostridium difficile, is a potential side effect of almost all antibacterial agents, including Keflex. Symptoms include persistent, watery and sometimes bloody diarrhea.
  • May increase the time it takes for blood to clot. People with liver or kidney disease, who are malnourished, receiving a long course of therapy, or already on anticoagulants are more at risk.
  • May interact with some drugs including metformin, probenecid, and some urinary glucose tests.

Notes: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. For a complete list of all side effects, click here.

4. Bottom Line

Keflex is a cephalosporin-type antibiotic that may be used in the treatment of gram-positive cocci infections. However, it should be used with caution in people with a penicillin allergy.

5. Tips

  • Can be taken with or without food.
  • Take exactly as directed and for the duration intended. Do not finish the course earlier than prescribed, even if you feel better because this encourages the growth of resistant bacteria. Do not take Keflex for any other infection other than the one you have been prescribed it for. Keflex will not treat viral infections such as the flu.
  • Seek medical advice if chronic diarrhea develops during or following a course of Keflex.
  • Talk to your doctor if you develop any worrying side effects after taking Keflex such as abdominal pain, excessive bruising or bleeding, or a rash.

6. Response and Effectiveness

  • Peak concentrations of Keflex are reached one hour after dosing; however, it may take up to 48 hours before infection-related symptoms start to abate. Keep taking Keflex for the total duration prescribed, even if you feel better.

7. Interactions

Medicines that interact with Keflex may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with Keflex. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with Keflex include:

  • anticoagulants (blood thinners), such as warfarin (may prolong bleeding time)
  • estradiol
  • metformin
  • other antibiotics, such as chloramphenicol, sulfonamides, macrolides, and tetracycline
  • probenecid
  • probiotics, such as Lactobacillus
  • sodium picosulfate
  • vaccinations, such as BCG, cholera, or typhoid vaccine (may diminish the effectiveness)
  • vitamins, such as fat-soluble vitamins A, D, E, or K, folate, iron, or zinc (may decrease blood concentrations of Keflex)

Note that this list is not all-inclusive and includes only common medications that may interact with Keflex. You should refer to the prescribing information for Keflex for a complete list of interactions.


Keflex (cephalexin) [Package Insert]. Revised 04/2019. Pragma Pharmaceuticals, LLC

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Keflex only for the indication prescribed.

Copyright 1996-2020 Revision date: April 8, 2020.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.