Birth control pills and antibiotics

This article originally appeared in the San Francisco Chronicle’s health section on August 8, 2012.

Myth: Taking antibiotics reduces the effectiveness of birth control pills.

Fact: Most prescription antibiotics and oral contraceptives come with safety inserts warning that taking the former may compromise the latter. There have even been court cases in which couples argued that doctors or drug companies were responsible for their unplanned pregnancies.

Research has shown that one drug, rifampin, may interfere with birth control. But for all other antibiotics, multiple studies have failed to find a definitive link between the drugs and birth control failures.

A 2011 Harvard study found no difference in oral birth control failure rates between women who took antibiotics in the four weeks before conception and those who did not. The study included more than 43,000 women, 1,330 of whom became pregnant while using oral contraceptives.

Two earlier papers, published in 1997 and 2002, also found no difference in the failure rates of birth control between women who were taking antibiotics and those who were not.

The one documented exception is rifampin, which is used to treat tuberculosis and, sometimes, meningitis. Studies have shown this drug changes the way a woman’s body interacts with oral contraception, weakening its effectiveness.

Researchers note that some of the misconceptions around birth control and antibiotic use may come from the fact that oral contraceptives already have a failure rate of 1 percent to 3 percent. So women who happen to be taking an antibiotic when they get pregnant while on oral contraception may have forgotten a pill or experienced some other birth control failure.

But despite the lack of evidence so far, doctors say that more research may be necessary to totally rule out the possibility that antibiotics affect birth control. Most of the studies done so far have been small, because not many women get pregnant while taking birth control pills and antibiotics at the same time. If there is a subtle but significant effect from antibiotics, it may take a larger sample size to identify that risk.

Concerned patients should talk with their doctors and take all medications exactly as directed. They might also consider a back up form of contraception, like a condom, in the month following antibiotic use.