a complex study published in may 2011 involved about 18 000 women and 1330 episodes of contraceptive failure. researchers looked at the data on women both during months of contraceptive success (i. e. not getting pregnant) versus contraceptive failure to see if taking antibiotics made any difference. the bottom line: contraceptive failures did occur but it was no more likely to occur if a woman was taking oral antibiotics. warning: it is well known that one particular antibiotic rifampin does definitely change the metabolism of many oral contraceptives. the study excluded women on that medication because the interaction of rifampin and contraceptives is well established. if you’re taking rifampin you must use a second form of birth control. even the best largest studies can’t say with 100% certainty that a super-rare event can’t occur. it could be that in very rare cases antibiotics could somehow affect the way oral contraceptives work. so if you want to be super-safe using two contraceptives is never a mistake.
do antibiotics decrease the effectiveness of oral contraceptives?