Interaction of doxycycline and oral contraceptives

American Family Physician,  June, 1991  

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Unexpected pregnancies have been reported in small numbers of women taking oral contraceptives in combination with several types of antibiotics, including rifampin, ampicillin, neomycin, tetracycline, oxytetracycline, cotrimoxazole, metronidazole, cephalosporins, trimethoprim, erythromycin, sulfonamides and griseofulvin. These antibiotics have been implicated as a primary reason for contraceptive failure. Because many young women receive doxycycline, a tetracycline derivative, in the treatment of sexually transmitted diseases and other infections, the potential interaction of doxycycline with oral contraceptives is important. Neely and colleagues conducted a controlled, clinical trial to evaluate the effects of doxycycline on concentrations of oral contraceptive hormones.

Twenty-four healthy young women who were receiving oral contraceptives from a university-based family planning clinic were recruited as volunteers for the study. All of the subjects had been taking a low-dose oral contraceptive for at least two months that contained 1 mg of norethindrone and 35 (micro)g of ethinyl estradiol. Compliance was ensured by pill counts. The study consisted of a control phase and a treatment phase. Serum samples were obtained on days 18, 19 and 20 of the cycle to measure levels of ethinyl estradiol, norethindrone and endogenous progesterone levels (control phase). During the following cycle, patients were given 100 mg of doxycycline orally twice daily, beginning on day 14 of the cycle, and blood samples were obtained on days 18, 19 and 20 of the cycle for repeat measurements of hormone levels (treatment phase).

No significant differences were found in serum hormone levels between the control phase and the treatment phase, indicating that doxycycline was unlikely to cause contraceptive failure. None of the patients had progesterone elevations suggesting ovulation in either cycle. Large interpatient and intrapatient variations in hormone levels were observed during the study period, indicating that large numbers of patients need to be evaluated to provide definitive proof that antibiotics do not interfere with the action of oral contraceptives. Nonetheless, the authors believe that pregnancies attributed to interaction of oral contraceptives and tetracyclines are actually due to other causes or represent a rare idiosyncratic response in individual women. (Obstetrics and Gynecology, March 1991, vol. 77, p. 416.) k

COPYRIGHT 1991 American Academy of Family Physicians
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