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There is evidence that the anticonvulsant action of phenytoin is antagonized by Folic Acid. A patient whose epilepsy is completely controlled by phenytoin may require increased doses to prevent convulsions if Folic Acid is given.
Folic deficiency may result from increased loss of folate, as in renal dialysis and/or interference with metabolism (e.g., Folic Acid antagonists such as methotrexate); the administration of anticonvulsants, such as diphenylhydantoin, primidone, and barbiturates; alcohol consumption and, especially, alcoholic cirrhosis; and the administration of pyrimethamine and nitrofurantoin.
False low serum and red cell folate levels may occur if the patient has been taking antibiotics, such as tetracycline, which suppress the growth of Lactobacillus casei.
