Antibiotic and oral contraceptive drug interactions: Is there a need for concern?

To assess the clinical significant of antibiotic and oral contraceptive drug interactions.

MEDLINE search from 1975 to 1998 (September) inclusive. Search terms 'antitiobic', 'oral contraceptive' and 'pregnancy' were included. Published papers as well as references from these papers were reviewed. Papers documenting mechanistic interactions between antibiotics and oral contraceptives were included.

Studies reporting oral contraceptive pharmacokinetics, mechanisms, incidence, implicated antibiotics and clinical consequences of antibiotic/oral contraceptive drug interactions.

Reports of oral contraceptive failure seem to be most numerous in women using preparations containing 30 μg of ethinylestradiol and 150 μ g of levonorgestrel. Rifampin is the only antibiotic that has been reported to reduce plasma estrogen concentrations. When taking rifampin, oral contraceptives cannot be relied upon and a second method of contraception is mandatory. Amoxicillin, ampicillin, griseofulvin, metronidazole and tetracycline have been associated with contraceptive failure in three or more clinical cases. When these agents are used, the clinician should discuss the available data with the patient and suggest a second form of birth control. Other antibiotics are most likely safe to use concomitantly with oral contraceptives.

Rifampin is the only antibiotic to date that has been reported to reduce plasma estrogen concentrations. Oral contraceptives cannot be relied upon for birth control while taking rifampin.