Psychiatric Drugs and Medications Q 29
When caring for a client who is receiving phenytoin and warfarin (Coumadin), the nurse would expect which of the following drug-drug interactions?
A. Decreased effectiveness of warfarin
B. Increased effectiveness of phenytoin
C. Increased effectiveness of warfarin.
D. Decreased effectiveness of phenytoin.
Correct Answer: A. Decreased effectiveness of warfarin
The interaction will reduce the effectiveness of warfarin. 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.
Option B: Using warfarin together with phenytoin may cause bleeding more easily. It may also increase phenytoin levels. Phenytoin levels and prothrombin time or International Normalized Ratio (INR) should be monitored whenever the dosage is changed or discontinued.
Option C: 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.
Option D: Despite the approval of new anticoagulants within the past several years, warfarin continues to be commonly used. Thus, awareness of drug interactions involving warfarin continues to be relevant. The impact of phenytoin on warfarin has been reported previously in the literature to potentiate the anticoagulant effect or interact in a biphasic manner.