Folate supplements and phenytoin-salicylate interaction

We found biphasic fluctuations of serum phenytoin level when aspirin was added to chronic phenytoin therapy for an epileptic patient. The total serum phenytoin level was lowered initially by addition of aspirin. However, after 4 months on phenytoin-aspirin combination therapy, he showed elevated serum phenytoin levels, mild nystagmus, and serum folate deficiency.