Which medication is NOT listed as reducing the effectiveness of estrogen-containing contraceptives?

• A. Enzyme-inducing anticonvulsants (carbamazepine, phenytoin, phenobarbital)
• B. Rifampin
• C. St. John's wort
• D. Acetaminophen

Answer: (D)

Estrogen-containing contraceptives rely on stable hormone levels to suppress ovulation. Medications that induce liver enzymes increase the metabolism and clearance of estrogen and progestin, lowering their plasma concentrations and undermining contraceptive protection.

Enzyme-inducing anticonvulsants like carbamazepine, phenytoin, and phenobarbital accelerate the breakdown of these hormones, increasing breakthrough bleeding and the risk of pregnancy. Rifampin is a strong inducer of hepatic enzymes as well, producing the same effect. St. John’s wort also induces enzymes that metabolize estrogens, reducing effectiveness.

Acetaminophen does not induce hepatic enzymes in a way that lowers estrogen levels, so it is not associated with decreased effectiveness of estrogen-containing contraceptives.

Clinical takeaway: when starting any enzyme-inducing medication, discuss backup contraception with a clinician, since the contraceptive effectiveness may be reduced during therapy and for a period after stopping.