Last week, while flying home for a high school reunion, I sat next to a very pleasant, talkative woman and her adorable and polite 8 year-old son. The woman and I started chatting and eventually I learned that she is an accountant, and I in turn shared that I am a doctor. Sometime later she non-chalantly explained that she had gotten pregnant at a young age while on oral contraceptives, because she was taking an antibiotic and was not warned by her physician that the antibiotic could make her oral contraceptive ineffective. I thought, “Really? Is this woman really telling me this?” and then, “Can that really happen?”

So of course I had to look into it.

A review of the literature shows there have been few controlled studies and no prospective randomized trials of the effects of antibiotics on combination oral contraceptives.  Most of the available data do not indicate any major reduction in the efficacy of oral contraceptives with concurrent common antibiotic use. For example, an article published in the Journal of the American

Academy of Dermatology in 1997 examined the effect of commonly prescribed oral antibiotics (tetracyclines, penicillins, cephalosporins) on the failure rate of oral contraceptives. The authors found that there was no significant increase in the failure rate of the oral contraceptives in the women taking the above-mentioned antibiotics.

One antibiotic however has clearly been shown in studies to decrease the effectiveness of birth control pills. That antibiotic is Rifampin, used to treat tuberculosis. Fortunately, Rifampin is not a commonly prescribed antibiotic, as tuberculosis is not very common in the U.S.

The belief that antibiotics could diminish the effectiveness of birth control arose after women noted that they got pregnant while on birth control and antibiotics concurrently.  Like all drugs, oral contraceptives are not 100% effective, with the failure rate in the U.S. reported to be as high as 3%. It is possible that the case reports of unintended pregnancies during antibiotic therapy may simply represent the normal failure rate of these drugs.  The limited number of studies show that the commonly prescribed antibiotics do not increase the failure rate of oral contraceptives by any significant amount (with the exception of Rifampin).  However, these studies cannot reliably exclude a small decrease in efficacy specifically in the “low-dose” (<35 g of estrogen) combination OCPs.

What exactly is the interaction between antibiotics and OCPs that would make the OCPs ineffective?

There are two ways that antibiotics may diminish the action of birth control pills. Birth control pills contain estrogens. Some antibiotics, e.g., Rifampin, cause the enzymes in the liver to increase the break-down of estrogens and can therefore decrease the levels of estrogens in the body and the effectiveness of the pills. This can result in unwanted pregnancy.

The other way that antibiotics can interfere with the effectiveness of birth control pills is by reducing the re-circulation of estrogens within the body. Estrogens, e.g., ethinyl estradiol in birth control pills, are broken-down by conversion in the liver to other chemicals, which are then secreted into the intestines. Bacteria in the intestine are able to convert these chemicals back into the active estrogen, which is then reabsorbed into the body and acts to prevent pregnancy. Theoretically, antibiotics can kill the bacteria that convert the inactive chemicals to the active estrogen, and, therefore, may interfere with the effectiveness of birth control pills. Unwanted pregnancies could occur. Since it is better to be safe than sorry, many sources advise women taking birth control pills to use a second reliable method of birth control when taking antibiotics.

Take home message: while most antibiotics do not increase the failure rate of oral contraceptive pills, to be on the safe side, when taking an antibiotic while on an oral contraceptive pill, use a barrier method during intercourse to prevent pregnancy.