Background Enzyme-inducing antibacterial drugs can impair the efficacy of hormonal contraceptives. Suspicion that other antibiotics might do likewise led to advice that extra contraceptive precautions should be taken during a course of antibiotics. However, current advice is that the purported interaction does not occur, based on small studies observing few pregnancies, assuming that all women are susceptible, and without measuring unbound hormone concentrations.

Objective To test the null hypothesis that antibiotics do not impair the effectiveness of oral contraceptives.

Design A database review of Yellow Card reports to the UK’s Medicines and Healthcare products Regulatory Agency.

Data Spontaneous reports of suspected adverse drug reactions in people taking antibacterial drugs (n=74 623), enzyme-inducing medicines (n=32 872), or control medicines (n=65 578).

Main outcome measures Reports of the primary outcome—unintended pregnancies; reports of cardiac arrhythmias and headaches (control events); reports of congenital abnormalities (positive control events); and reports of diarrhoea (a possible confounding factor).

Results Compared with control medicines, unintended pregnancies were seven times more commonly reported with antibiotics and 13 times more commonly reported with enzyme inducers (the positive controls). Congenital abnormalities were reported seven times more often with enzyme inducers but were not more common with antibiotics. Diarrhoea was not a confounding factor.

Conclusion This study provides a signal that antibacterial drugs may reduce the efficacy of hormonal contraceptives. Women taking hormonal contraceptives should be warned that antibiotics may impair their effectiveness. Extra precautions can be taken during a course of antibiotics; an unintended pregnancy is a life-changing event.