Can cephalexin (Keflex) cause neurologic issues?

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Last updated: October 15, 2026 • View editorial policy

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Can Keflex Cause Neurologic Issues?

Yes, cephalexin (Keflex) can cause neurologic issues, though this is uncommon and occurs primarily in specific high-risk situations, particularly in patients with renal impairment or when dosing is not appropriately adjusted.

Evidence from FDA Drug Label

The FDA-approved prescribing information for Keflex explicitly states that "several cephalosporins have been implicated in triggering seizures, particularly in patients with renal impairment when the dosage was not reduced" 1. The label also documents other CNS effects including:

  • Dizziness
  • Agitation
  • Confusion
  • Hallucinations

The FDA guidance is clear: "If seizures associated with drug therapy should occur, the drug should be discontinued. Anticonvulsant therapy can be given if clinically indicated" 1. Additionally, the label emphasizes that "Keflex should be administered with caution in the presence of markedly impaired renal function" 1.

Clinical Context and Risk Factors

While cephalexin can cause neurotoxicity, it's important to understand the relative risk compared to other cephalosporins. Based on comparative data, cephalexin has lower pro-convulsive activity than many other cephalosporins. The relative convulsing activity (with penicillin G = 100 as reference) shows cephalexin is not among the highest-risk agents 2:

  • Cefazolin: 294
  • Cefepime: 160
  • Cephalexin: Not listed among the highest-risk agents
  • Meropenem: 16

Primary Risk Factors for Neurotoxicity:

  1. Renal impairment - The most critical risk factor, as cephalosporins accumulate when clearance is reduced
  2. Inadequate dose adjustment - Failure to reduce dosing frequency or amount in renal dysfunction
  3. Young age with developmental issues - A case report documented delirium in a 16-year-old with developmental delay and prior head injury 3
  4. Pre-existing CNS conditions - History of seizures or other neurologic disorders

Spectrum of Neurologic Manifestations

A comprehensive French pharmacovigilance analysis of cephalosporin CNS adverse effects (though primarily involving other cephalosporins) documented the following presentations 4:

  • Encephalopathy (30.3%)
  • Confusional state (19.4%)
  • Convulsions/seizures (15.1%)
  • Myoclonus (9.4%)
  • Status epilepticus (9.2%)
  • Coma (6.3%)
  • Hallucinations (4.3%)

The mean time to onset was 7.7 days with a mean duration of 6 days 4.

Clinical Approach When Neurotoxicity is Suspected

If unexplained neurologic symptoms develop in a patient taking cephalexin:

  1. Immediately assess renal function - Check creatinine clearance, as this is the most common underlying cause
  2. Consider drug plasma levels if available - Though not routinely done for cephalexin, 57.9% of elevated cephalosporin levels in one study were related to renal impairment 4
  3. Obtain EEG if symptoms are significant - 81% of patients with cephalosporin neurotoxicity had abnormal EEGs 4
  4. Discontinue cephalexin immediately if neurotoxicity is suspected
  5. Provide supportive care including anticonvulsants if seizures occur 1

Important Caveats

  • Neurotoxicity can occur even with normal renal function, though this is rare. A recent case report documented cefepime-induced encephalopathy in a patient with intact kidney function 5, highlighting that renal impairment is not an absolute prerequisite
  • The risk-benefit ratio changes when free drug concentrations exceed 8 times the MIC, where neurotoxicity risk increases substantially 2
  • Most patients tolerate cephalexin well - In a recent real-world study of 230 patients, only 0.9% of URTI patients and 5.2% of skin infection patients experienced any adverse events, with no serious adverse events reported 6

Bottom Line for Clinical Practice

Cephalexin can cause neurologic issues but does so infrequently when dosed appropriately. The key is recognizing high-risk patients (primarily those with renal impairment) and adjusting doses accordingly. When unexplained neurologic symptoms emerge during cephalexin therapy, immediately evaluate renal function, discontinue the drug, and provide supportive care. The neurotoxicity is typically reversible upon drug discontinuation.

References

3
Research

Delirium in an adolescent patient during treatment with cephalexin.

The Journal of adolescent health : official publication of the Society for Adolescent Medicine, 2006

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