The effect of tetracycline on levels of oral contraceptives.

Despite the widespread use of tetracycline for treatment of dermatologic disorders and sexually transmitted diseases, the pharmacodynamics of oral contraceptive use in the presence of tetracycline has not been studied. Seven normal women ingested an oral contraceptive containing ethinyl estradiol 35 micrograms and norethindrone 1 mg in the follicular phase of the menstrual cycle. On day 0 baseline ethinyl estradiol and norethindrone levels were obtained at 0, 1/2, 3/4, 1, 2, 4, 12, and 24 hours after oral contraceptive administration. On day 1 tetracycline 500 mg was given orally every 6 hours while the oral contraceptive was continued. Tetracycline, ethinyl estradiol, and norethindrone levels were determined at the same time intervals as on day 0. Oral contraceptive and tetracycline were continued for up to 10 days, and additional concentrations of ethinyl estradiol, norethindrone, and tetracycline were determined between days 5 and 10. Four additional normal women ingested tetracycline for 5 to 10 days. Tetracycline levels were determined at the time intervals noted above on day 1 and days 5 to 10. No significant decrease in plasma ethinyl estradiol or norethindrone concentration was seen with either short-term (24 hours) or long-term (5 to 10 days) ingestion of tetracycline. Similarly, levels of tetracycline do not significantly decrease with ingestion of a low-dose oral contraceptive.