What is Keflex (cephalexin), and how is it used?

Keflex (cephalexin) is a cephalosporin antibiotic used to treat infections caused by bacteria including middle ear infections (otitis media), tonsillitis, throat infections, laryngitis, bronchitis, pneumonia, urinary tract (UTIs), and skin and bone infections. Common side effects of Keflex include

Serious side effects of Keflex include mild or severe cases of pseudomembranous colitis (a mild to severe inflammation of the colon), Clostridium difficile (C. diff) infection, and allergic reactions. Serious but rare allergic reactions include seizures, severe allergic reactions (anaphylaxis), and low platelet or red blood cell count.

Drug interactions of Keflex include BCG and typhoid vaccines. There are no adequate and well-controlled studies of Keflex in pregnant women. Keflex should be used during pregnancy only if clearly needed. Keflex is excreted in breast milk. Keflex should be used with caution or stopped when breastfeeding.

What are the important side effects of Keflex (cephalexin)?

The most common side effects of cephalexin are:

Individuals who are allergic to penicillin may also be allergic to cephalexin. Serious but rare reactions include seizures, severe allergic reactions (anaphylaxis), and low platelet or red blood cell count.

Cephalexin, like almost all antibiotics, may cause mild or severe cases of pseudomembranous colitis, a mild to severe inflammation of the colon. Antibiotics, including cephalexin alter the types of bacteria in the colon and permit overgrowth of a bacterium called Clostridium difficile. Studies indicate that toxins produced by Clostridium difficile are a primary cause of pseudomembranous colitis.

Keflex (cephalexin) side effects list for healthcare professionals

The following serious events are described in greater detail in the Warning and Precautions section:

  • Hypersensitivity reactions
  • Clostridium difficile-associated diarrhea
  • Direct Coombs' Test Seroconversion
  • Seizure Potential
  • Effect on Prothrombin Activity
  • Development of Drug-Resistant Bacteria

Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice

In clinical trials, the most frequent adverse reaction was diarrhea. Nausea and vomiting, dyspepsia, gastritis, and abdominal pain have also occurred. As with penicillins and other cephalosporins, transient hepatitis and cholestatic jaundice have been reported.

Other reactions have included hypersensitivity reactions, genital and anal pruritus, genital candidiasis, vaginitis and vaginal discharge, dizziness, fatigue, headache, agitation, confusion, hallucinations, arthralgia, arthritis, and joint disorder. Reversible interstitial nephritis has been reported. Eosinophilia, neutropenia, thrombocytopenia, hemolytic anemia, and slight elevations in aspartate transaminase (AST) and alanine transaminase (ALT) have been reported.

In addition to the adverse reactions listed above that have been observed in patients treated with Keflex, the following adverse reactions and other altered laboratory tests have been reported for cephalosporin class antibacterial drugs:

Other Adverse Reactions

Fever, colitis, aplastic anemia, hemorrhage, renal dysfunction, and toxic nephropathy.

Altered Laboratory Tests

Prolonged prothrombin time, increased blood urea nitrogen (BUN), increased creatinine, elevated alkaline phosphatase, elevated bilirubin, elevated lactate dehydrogenase (LDH), pancytopenia, leukopenia, and agranulocytosis.

What drugs interact with Keflex (cephalexin)?


Administration of Keflex with metformin results in increased plasma metformin concentrations and decreased renal clearance of metformin.

Careful patient monitoring and dose adjustment of metformin is recommended in patients concomitantly taking Keflex and metformin.


The renal excretion of Keflex is inhibited by probenecid. Co-administration of probenecid with Keflex is not recommended.

Interaction With Laboratory Or Diagnostic Testing

A false-positive reaction may occur when testing for the presence of glucose in the urine using Benedict's solution or Fehling's solution.

Treatment & Diagnosis

Medications & Supplements

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Medically Reviewed on 2/13/2020
FDA Prescribing Information

Professional side effects list and drug interactions sections courtesy of the U.S. Food and Drug Administration