 [ Symptoms ] do antibiotics decrease the effectiveness of oral contraceptives [ Disease ] 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 are taking rifampin you must use a second form of birth control even the best largest studies can not say with 100 certainty that a super rare event can not 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