Patients taking oral contraceptives do not need to use a backup method of contraception when taking which of the following medications?

• A. Ampicillin
• B. Ketoconazole
• C. Phenytoin (Dilantin)
• D. Metformin (Glucophage)

Answer: (D)

The rationale for selecting metformin as the correct answer is based on its mechanism of action and interactions with hormonal contraceptives. Metformin is an oral antidiabetic medication primarily used to manage blood sugar levels in patients with type 2 diabetes. It does not affect the metabolism or efficacy of oral contraceptives. Therefore, patients taking metformin can continue to rely on their oral contraceptives for effective pregnancy prevention without the need for a backup method.

In contrast, other medications listed have been shown to potentially reduce the effectiveness of oral contraceptives. For instance, antibiotics like ampicillin may alter the gut flora and impact the enterohepatic circulation of contraceptive hormones, although this is somewhat controversial. Ketoconazole, an antifungal, can affect the metabolism of estrogen and progesterone, possibly leading to a reduced efficacy of oral contraceptives. Phenytoin, an anticonvulsant, is known to induce liver enzymes, which can accelerate the metabolism of the hormones in birth control pills, necessitating a backup contraceptive method.

Thus, among the choices presented, metformin stands out as the medication that does not require additional contraceptive measures for those who are already using oral contraceptives.