Which drug's effects are NOT increased by oral contraceptives?

The effects of phenytoin are not increased by oral contraceptives due to the nature of its metabolism and the interactions involved. Phenytoin is an anticonvulsant that is primarily metabolized in the liver via the cytochrome P450 enzyme system. Oral contraceptives can induce certain liver enzymes, leading to increased metabolism of some drugs, but phenytoin is an exception as it can have a complex relationship with hepatic enzymes. In fact, while oral contraceptives may increase the metabolism of some drugs, they can lead to a decrease in the effectiveness of phenytoin, as it tends to be eliminated more quickly when enzyme induction occurs.

In contrast, the other medications listed are affected by the presence of oral contraceptives. Warfarin, an anticoagulant, may have altered effects due to changes in liver metabolism and potential interactions that can enhance anticoagulant effects. Metformin, used for managing blood glucose levels in type 2 diabetes, may be influenced by alterations in metabolism and absorption due to hormonal changes from oral contraceptives. Simvastatin, a cholesterol-lowering medication, may also have increased effects as oral contraceptives can impact its metabolism, potentially raising the risk of side effects