Antiepileptic drugs and warfarin.

In a recent CMAJ commentary on drug interactions with warfarin, David Juurlink overlooked a very important category of drugs that interact with warfarin: antiepileptic drugs. Phenytoin, carbamazepine and phenobarbital are potent inducers of the cytochrome P450 system, and their interactions with warfarin have been known for decades. These drugs can substantially increase the rate at which warfarin is metabolized and thus reduce the effect of a previously adjusted dose. Likewise, sudden withdrawal of any of these drugs may decrease the rate at which warfarin is metabolized and put a patient taking a combination of these drugs at an increased risk of bleeding. Antiepileptic drugs are not only prescribed for epilepsy, which is estimated to affect 200 000 Canadians; they are also used to treat psychiatric disorders in a large population of patients. I suggest that Juurlink should add antiepileptic drugs as a ninth category to Table 1.