Phenytoin serum levels can be raised by
co-trimoxazole (which contains trimetho- prim).
Trimethoprim can increase decrease the clearance of
phenytoin by 30% and prolong its
half-life.
Phenytoin toxicity may develop in some cases. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
The risk of
toxicity is small and is most likely in those with
serum phenytoin levels at the top end of the range. Monitor
phenytoin levels and adjust the dose accordingly. Indicators of
phenytoin toxicity include blurred vision, nystagmus, ataxia or drowsiness.