Provide an example of a significant drug interaction with antiseizure medications.

• A. Phenytoin induces hepatic enzymes reducing levels of oral contraceptives and many other drugs.
• B. Valproate enhances the clearance of all antiseizure medications.
• C. Lamotrigine inhibits liver enzymes causing high levels of other drugs.
• D. Levetiracetam drastically increases warfarin absorption.

Answer: (A)

Enzyme induction by a seizure medication speeds up the body's drug‑metabolizing enzymes, which lowers the blood levels of other drugs that rely on those enzymes for clearance. Phenytoin is a classic hepatic enzyme inducer (affecting enzymes like CYP450 and UGTs). Because it ramps up metabolism, it reduces the plasma concentrations of many coadministered drugs, including oral contraceptives. This can lead to reduced contraceptive effectiveness and potential pregnancy, illustrating a significant and clinically important interaction that requires attention, monitoring, or adjustment.

Other options don’t fit this pattern. Valproate tends to inhibit, not induce, liver enzymes, which can raise levels of certain drugs rather than universally increasing their clearance. Lamotrigine’s interactions aren’t due to broad enzyme inhibition causing high levels of other drugs; its clinically relevant interactions involve specific combinations (like with valproate) and dosing considerations. Levetiracetam generally has minimal drug interactions and does not markedly increase warfarin absorption.