Cephalexin Side Effects | 5 Known Side Effects of Cephalexin

Cephalexin Side Effects

Cephalexin is the antibiotic of choice that effectively treats bacterial respiratory infections, ear infections, or lower urinary tract infections. It is a well-established antibacterial that has been safely used for over 50 years. (1)

While you hardly wait to feel better and complete the course, suddenly, you experience diarrhea, nausea, or vomiting. If not the first antibiotic treatment, you know those side effects might occur. However, diarrhea can be watery, comes a few weeks after finishing treatment, and lasts for more than a week. If so, this is a sign that you need medical care. (1)

Diarrhea and other side effects associated with Cephalexin, although rarely happen, can be severe or life-threatening; therefore, they require immediate action. (1) Keep reading this article to find Cephalexin’s side effects and learn when to call your doctor or stop using the drug immediately.

What are the possible side effects of Cephalexin?

Like any other drug, Cephalexin can cause adverse reactions when taken at the recommended doses. The most common possible side effects are nausea, vomiting, diarrhea, dyspepsia, and abdominal pain, which might occur in up to 10% of the patients. (1, 2)

Although the following side effects don’t happen very often, they are severe and require medical help; their frequency of occurrence is less than 0,1% of cases. (2) For this reason, those possible side effects are listed in the section Warning and Precautions of the Prescribing Information. (1)

Serious hypersensitivity reactions

Cephalexin, like any other drug substance, may cause an allergic reaction. It is usually manifested with skin rash, itching, swelling, and anaphylaxis (wheezing, breathing difficulty, and sometimes nausea, vomiting, and diarrhea). (1, 3)

If you have had an allergic reaction while using penicillin’s antibiotics, the chances of being allergic to Cephalexin are up to 10%. Therefore, tell your doctor about your previous allergic reaction so they can make the right choice when prescribing an antibiotic for you. (1)

If you experience any allergic symptoms, stop using Cephalexin and ask for medical care. (1)

Clostridium difficile-associated diarrhea

The use of Cephalexin or another antibiotic alerts bacteria that normally live in our bodies. Antibiotic kills good bacteria, while harmful bacteria such as Clostridium difficile overgrowth. Clostridium difficile produces toxins that may cause diarrhea named Clostridium difficile-associated diarrhea (CDAD). The diarrhea is watery, followed by nausea, dehydration, abdominal cramping and pain, rapid heart rate, fever, loss of appetite, and distended abdomen. (1, 4)

When severe, CDAD can be life-threatening and requires treatment with a specific antibiotic that might include hospital medical care. Be aware that CDAD might occur even two months after antibiotic treatment; therefore, don’t underestimate and self-medicate. Call your doctor carefully to examine and confirm if CDAD is suspected. (1)


Some drugs may induce acute intravascular hemolysis. It is a sudden destruction of the red blood cell in the blood vessels and results in anemia. When the red blood cell level becomes too low, fatigue, dizziness, heart palpitations, shortness of breath, and other symptoms will occur. (1, 5)

If it happens while on cephalexin therapy, the doctor will change antibacterial therapy and, if needed, give you additional anemia therapy. (1)

Seizure Potential

In patients who have renal impairment and the antibiotic dose is not reduced, Cephalexin or some other cephalosporin might trigger seizures. If it happens, cephalexin therapy should be discontinued, and the doctor might give another drug for the seizure if needed. (1)

Effect on Prothrombin Activity

Cephalexin may prolong prothrombin time (the time blood takes to form a clot) in specific groups of patients (renal or hepatic impairment, patients with poor nutritional state, patients at prolonged antibacterial therapy, and patients on anticoagulant medicine). In those cases, the doctor will require monitoring of the prothrombin time. (1, 6)

Development of Drug-Resistant Bacteria

Doctors prescribe Cephalexin when there is proof of bacterial infection. However, if the antibiotic is not used correctly, bacteria develop a different defensive mechanism, so they become able to defeat the antibiotic. Once bacteria become resistant, it is harder to be treated because antibiotic choice is limited. (1, 7)

Therefore, take the complete cephalexin course exactly as your doctor prescribed, even if you feel better after a few days of initiating the treatment. If you skip the dose or discontinue treatment earlier, it will decrease cephalexin effectiveness and increase the probability of developing resistance. (1)

Check below the recommendations for cephalexin usage.

Interactions with other drugs and supplements

Cephalexin may interact with other drugs or substances, which affects its effectiveness or vice versa.

Cephalexin increases metformin concentration in plasma and decreases its elimination through the kidney. For this reason, tell your doctor that you take metformin to adjust its dose as needed. (1)

Probenecid is a drug used in gout and gouty arthritis that helps the kidney eliminate uric acid from the body. Probenecid inhibits the renal excretion of Cephalexin; therefore, probenecid and Cephalexin should not be used together. (1, 8)

Cephalexin interacts with some of the glucose urine tests using specific solutions. The result of such interaction is a false positive reaction. If you need to check the glucose level, tell the healthcare professional you are on cephalexin treatment. (1)

Cephalexin can decrease the efficacy of the BCG, typhoid, and Vibrio cholerae CVD 103-HgR strain live antigen, so the doctor should be aware if you or your child is taking Cephalexin before moving on vaccination. (9)

Zinc is a mineral that our bodies need for about 100 enzymes to play their role in the body and helps cells to grow and multiply. However, zinc decreases cephalexin absorption, resulting in lower concentration in serum and decreased cephalexin efficacy. (9)

Due to this, the United Kingdom National Health Service recommends taking zinc supplements three hours after the cephalexin dose. (2, 10)

Why and how to use Cephalexin

Cephalexin is an antibiotic that kills bacteria and is used to treat infection caused by any of the following bacteria: Staphylococcus aureus, Streptococcus pneumonia, Streptococcus pyogenes, Escherichia coli, Haemophilus influenzae, Klebsiella pneumoniae, Moraxella catarrhalis, and Proteus mirabilis. (1)

It is available in capsules, tablets, and oral solutions to treat pneumonia and other respiratory tract infections, ears, skin, bone, genital, and urinary tract infections. (1)

Adults and children older than one year can use Cephalexin. In adults and children over 15, 250 mg every six hours or 500 mg every 12 hours are recommended for 7-14 days. In pediatrics 1-15 years of age, 25-100 mg/kg is recommended depending on the type of bacteria and severity of infections. (1)

If your child uses cephalexin oral solution, keep the bottle in the fridge for 14 days.

Use Cephalexin as long as your doctor recommends, even if you feel better after a few days. Take the prescribed dose at the same interval. If you forgot, take it as soon as possible unless it is too close for the next dose. (1)

If you take Cephalexin more than the doctor prescribed and experience nausea, vomiting, diarrhea, or hematuria, it could be a sign of an overdose. If so, call the poison control helpline at 1-800-222-1222. (1, 11)

You should use Cephalexin if you’re pregnant or breastfeeding when the doctor decides it is needed. There is no clinical proof that Cephalexin hurts the fetus, but it is known that it is excreted in milk. Therefore, tell your doctor if you are pregnant or breastfeeding your child. (1)


Cephalexin has been available for over 50 years to treat bacterial infections with an acceptable safety profile. Side effects are well-known, and most of them are temporary and manageable.

You should pay attention when diarrhea occurs, you experience any allergy and anemia signs, or you get seizures. In this case, call your doctor immediately.

Whether severe or not, you can report any suspected adverse reaction to Food and Drug Administration at 1-800-FDA-1088 or www.fda.gov/medwatch.

Use Cephalexin as your doctor prescribed, each day at the same time and as long as needed. That is the right way to eliminate bacteria that caused your infection, keep it away from resistance, and have it effective for more years.

See Also

Ibuprofen Side Effects

Losartan Side Effects

Amoxicillin Side Effects

Amlodipine Side Effects

Sertraline Side Effects

Doxycycline Side Effects

  1. Food and Drug Administration, Keflex, Cephalexin capsules, Prescribing information, 2018 Dec. https://www.accessdata.fda.gov/drugsatfda_docs/label/2018/050405s107lbl.pdf
  2. NHS National Health Service, Cefalexin, 2020 Mar. https://www.nhs.uk/medicines/cefalexin/
  3. Harvard Medical School, Anaphylaxis: An overwhelming allergic reaction, 2009 Sept. https://www.health.harvard.edu/diseases-and-conditions/Anaphylaxis-An-overwhelming-allergic-reaction
  4. Mayo Clinic, C. difficile infection, 2021 Aug. https://www.mayoclinic.org/diseases-conditions/c-difficile/symptoms-causes/syc-20351691
  5. Cleveland Clinic, Hemolysis, 2022 Aug. https://my.clevelandclinic.org/health/diseases/24108-hemolysis
  6. Cleveland Clinic, Prothrombin Time (PT) Test, 2022 Mar. https://my.clevelandclinic.org/health/diagnostics/17691-prothrombin-time-pt-test
  7. CDC Centers for Disease Control and Prevention, Antimicrobial Resistance, 2022 Oct. https://www.cdc.gov/drugresistance/about/how-resistance-happens.html
  8. MedlinePlus, Probenecid, 2017 Jul. https://medlineplus.gov/druginfo/meds/a682395.html
  9. DRUGBANK online, Cephalexin, 2023 Jan, https://go.drugbank.com/drugs/DB00567
  10. Yi Ding, Yan-Yan Jia, Fan Li, Wen-Xing Liu, Cheng-Tao Lu, et al., The effect of staggered administration of zinc sulfate on the pharmacokinetics of oral cephalexin, Br J Clin Pharmacol. 2012 Mar; 73(3): 422–427. 2011 Oct https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3370346/
  11. MedlinePlus, Cephalexin, 2016 Jun https://medlineplus.gov/druginfo/meds/a682733.html

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