Final answer:
The nurse should inform the client that phenytoin can decrease the effectiveness of birth control pills.
Step-by-step explanation:
The nurse should inform the client that phenytoin can decrease the effectiveness of birth control pills. Phenytoin is an enzyme-inducing antiepileptic medication that can accelerate the metabolism of hormonal contraceptives, potentially reducing their efficacy. The client should be advised to use an additional form of contraception, such as condoms, while taking phenytoin to prevent unplanned pregnancy. It is crucial for the client to consult with their healthcare provider for personalized advice and guidance.