It says it right there in the package insert of almost any antibiotic, and just about any oral contraceptive pills: don’t mix the two. Taking antibiotics, it is said, can decrease the effectiveness of oral contraceptives. But is that actually true? It took some fancy science to add to the growing body of evidence that it’s all a myth.
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.
Big 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.
Now, it’s impossible with science to prove a negative. 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. But as far as could be determined by this large epidemiologic study, women on contraceptives (excluding rifampin) were no more likely to experience a contraceptive failure than women not taking antibiotics.
So why exactly is that warning on the labels of contraceptive pills and antibiotics? Numerous prior studies (here and here for example) have also attested to the lack of genuine evidence for any problem. But case reports of contraceptive failure have popped up, and no one wants to risk that — or any potential litigation that might ensue. So the warnings stand, despite any real evidence that they’re necessary.
If you want to make sure your oral contraceptive pills work well, avoid the common causes of failure. Take them every single day, as directed. Start the new pack exactly when you’re supposed to. Be careful when taking medications that do genuinely interact with contraceptive pills (this includes some epilepsy medications, the antibiotic rifampin, and the anti-fungal medicine griseofulvin.) Make sure whatever doc prescribes anything knows all of the medications you’re on to check for any potential interactions. Also, speak with your doctor about using emergency back-up contraception (a “morning after” pill) if you do find that you’ve skipped pills or otherwise compromised your contraception plan.