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Cephalexin: 7 things you should know

Medically reviewed by Carmen Pope, BPharm. Last updated on Aug 29, 2023.

1. How it works

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

2. Upsides

  • Treats a wide range of infections such as those occurring in the respiratory tract, ear, genitourinary area, bone, or on the skin.
  • Cephalexin has excellent activity against gram-positive staphylococci and streptococci bacteria, including susceptible isolates of Staphylococcus aureus, Streptococcus pneumoniae, S. pyrogens, Haemophilus influenzae, Klebsiella pneumoniae, Moraxella catarrhalis, and Proteus mirabilis.
  • May be given twice daily.
  • The usual dosage of cephalexin is 250mg every 6 hours but a dose of 500mg every 12 hours may also be administered. Larger dosages may be administered for more severe infections. Treatment is usually given for 7 to 14 days.
  • May be given with or without food.
  • Cephalexin is available as a generic.

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 are the most commonly reported. May alter some laboratory tests. Seizures have been reported rarely.
  • About 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.
  • Positive direct Coombs' tests have been reported during treatment with cephalosporin antibacterials, such as cephalexin.
  • The dosage of cephalexin 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 cephalexin. 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.

Note: 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. View complete list of side effects

4. Bottom Line

Cephalexin is a cephalosporin-type antibiotic that may be used in the treatment of infections caused by susceptible gram-positive bacteria. Approximately 10% of people who are allergic to penicillin will be allergic to cephalexin.

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 cephalexin for any other infection other than the one you have been prescribed it for. Cephalexin will not treat viral infections such as the flu.
  • Seek medical advice if chronic diarrhea develops during or within a few months of finishing a course of cephalexin.
  • Talk to your doctor if you develop any worrying side effects after taking cephalexin such as abdominal pain, excessive bruising or bleeding, or a rash.

6. Response and effectiveness

  • Peak concentrations of cephalexin are reached one hour after dosing; however, it may take up to 48 hours before infection-related symptoms start to abate.

7. Interactions

Medicines that interact with cephalexin may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with cephalexin. 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 cephalexin 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 cephalexin)

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


Further information

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

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

Copyright 1996-2023 Revision date: August 29, 2023.