Phenytoin-folic acid interaction: A lesson to be learned

Hanna Seligmann, Israel Potasman, Boaz Weller, Miguel Schwartz, Miron Prokocimer

Research output: Contribution to journalArticlepeer-review


A case of a patient who developed symptomatic phenytoin-induced folic acid deficiency is reported. Folate supplementation of 5 mg/d was followed by a decrease of serum phenytoin concentration to a subtherapeutic level with a breakthrough seizure. Estimation of phenytoin's Km-Vmax Michaelis-Menten pharmacokinetic parameters in this patient demonstrated that folate supplements indeed caused a significant decrease in the Km value. This decrease correlates with a greater affinity of the metabolizing hepatic enzymes for the drug, and hence, with the resultant increase in phenytoin's metabolism and decrease of its serum concentration and anticonvulsive effect. In an era of increasing knowledge of folate's pivotal role in various diseases, we call attention to this drug-vitamin interaction, and to the previously suggested recommendation that folate supplementation should be initiated whenever phenytoin therapy commences. Because folic acid dosages as low as 1 mg/d may perturbate phenytoin's metabolism, smaller deficiency preventive doses may be the advisable allowance for phenytoin-treated patients with normal pretreatment folate levels. This suggestion must be confirmed by a prospective study in a large cohort of patients.

Original languageEnglish
Pages (from-to)268-272
Number of pages5
JournalClinical Neuropharmacology
Issue number5
StatePublished - Sep 1999
Externally publishedYes


  • Drug-drug interaction
  • Folic acid
  • Phenytoin
  • Seizure


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