American Board of Pediatric Dentistry (ABPD) Oral Boards Practice Exam

When combined oral contraceptives rely on stable circulating levels of estrogen and progestin to suppress ovulation, certain antibiotics can disrupt those levels and raise the risk of breakthrough ovulation. Rifampin is a well-known enzyme inducer that accelerates the metabolism and clearance of ethinyl estradiol and the progestin, lowering their concentrations and potentially allowing ovulation to occur despite OC use.

Tetracyclines and penicillin derivatives have also been taught to affect OC effectiveness, though the evidence is less uniform. Their impact is thought to involve changes in gut flora that alter enterohepatic recirculation of estrogens, which can reduce estrogen levels enough to compromise contraception in some cases. Because of these interactions, it’s advisable to use backup contraception or a nonhormonal method during the antibiotic course and for a short period afterward.