Lack of effect of cholestyramine on phenytoin bioavailability

The effect of cholestyramine (4 g qid for 5 days) on the kinetics of phenytoin (400 mg orally) was investigated in normal subjects. Apart from a trend toward faster phenytoin absorption, the serum level profile of the drug during concurrent cholestyramine coadministration was similar to that observed in a control session. Areas under the serum phenytoin concentration curves were virtually identical in the two occasions. It is concluded that cholestyramine does not significantly affect the bioavailability of a single dose of phenytoin.