Drug interactions between amoxicillin and ethinyl estradiol

ADDITIONAL CONTRACEPTION RECOMMENDED: The effectiveness of estrogen-containing oral contraceptives may be impaired by concomitant treatment with antimicrobial agents. However, the risk appears to be small, and supportive data are primarily limited to anecdotal evidence from case reports and findings from uncontrolled or poorly controlled studies. Most antimicrobials, with the exception of the rifamycins and possibly griseofulvin, do not induce hepatic enzymes and have not been shown to significantly increase the clearance of oral contraceptive estrogens. Some investigators believe that antimicrobials interfere with the enterohepatic recirculation of estrogens by decreasing bacterial hydrolytic enzymes in the gastrointestinal tract that are responsible for regenerating parent estrogen molecules following first-pass metabolism. It is possible that a small number of women may be more susceptible to contraceptive failure and, consequently, are more sensitive to the effects of antimicrobials on estrogen disposition in vivo, but risk factors or genetic predispositions have yet to be identified.

MANAGEMENT: Until further data are available, women using oral contraceptives should be advised of the risk of breakthrough bleeding and unintended pregnancy during concomitant antimicrobial therapy. Alternative or additional methods of birth control should be considered during and for at least one week beyond the last dose of short-term antimicrobial therapy, and for at least the initial weeks of long-term antimicrobial therapy when risk may be the greatest.