Effect of Cimetidine on Phenytoin Serum Levels

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Abstract
Cimetidine [an antiulcer drug], 300 mg orally 4 times/day, was administered for 5 days to 9 epileptics who were stabilized while receiving phenytoin. Patients [5] had statistically significant increases in phenytoin serum levels, including 2 who became clinically toxic. One patient had a statistically significant decrease in phenytoin serum concentration. A relationship was not found between cimetidine levels and change in phenytoin serum levels. Cimetidine can cause significant changes in phenytoin serum levels which may be manifested clinically.