Phenytoin-Dexamethasone: A Possible Drug-Drug Interaction

Douglas D. Wong, Pharm D; R. Gerald Longenecker, Pharm D; Marcia Liepman, MD; Stephen Baker, MS; Monique LaVergne
JAMA. 1985;254(15):2062-2063. doi:10.1001/jama.1985.03360150038012.
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Phenytoin and dexamethasone are administered prophylactically in combination to prevent potential central nervous system complications seen in patients with head trauma and metastatic brain disease. Two different types of drug interactions have been reported to occur between phenytoin and dexamethasone. Liver enzyme stimulation by phenytoin may enhance dexamethasone's elimination, thereby reducing its steroidal effect.1,2 Lawson et al3 reported that serum concentrations of phenytoin were elevated in patients receiving concurrent dexamethasone therapy.3

We report our observations from a retrospective study of patients receiving both drugs. Unlike Lawson et al our findings indicate that phenytoin concentrations are routinely lowered when dexamethasone is concurrently prescribed. In the patients studied, phenytoin dosages had to be increased to maintain therapeutic serum concentrations (10 to 20 μg/mL).

Patients and Methods.—  For a three-month period we identified all patients who received a combination of phenytoin and dexamethasone. Forty patient records were reviewed and data


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