In a perfect world—aka, a laboratory setting—the Pill provides an almost impeccable barrier, preventing women from becoming pregnant an impressive 99.7 percent of the time.But we don’t live in a perfect world.In reality, oral contraceptives are sometimes taken hours or even days late when we finally remember. As a result, the Pill has a typical failure rate of about 9 percent. In rare cases, the Pill’s effectiveness can also be undermined by other medications or supplements. Understanding the things that do and do not affect the Pill’s ability to prevent pregnancy can help you avoid becoming part of that statistic.
1. Skipping pills. This is the most sure-fire way to impact the Pill’s effectiveness. The birth control pill, like all hormonal contraceptives, works by suppressing hormones that a woman’s body normally produces to regulate and trigger ovulation. Most birth control pills use a combination of the hormones estrogen and progestin to prevent ovulation. Skipping a pill causes those hormone levels to drop, and women don’t always avoid sex or use back-up birth control methods when that happens. In one study, around half of the participants accidentally skipped two or more pills in a row, but only 17.5 percent avoided sex for the following week and a mere 3 percent used a condom or another back-up method. “When you quit using birth control, your ovaries go right back to work,” says Philip Darney, M.D., distinguished professor of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco.
2. Not taking the Pill on time. Scientists have made oral contraceptives safer and reduced side effects by lowering hormonal doses, but that also means it’s more important than ever to take the Pill at approximately the same time every day. For progesterone-estrogen pills, you’ve got a window of plus-or-minus six to 12 hours, while progesterone-only pill users must be even more consistent about taking the Pill at the same time every day. “You’re counting on a steady state of hormones, and if you’ve been without active hormones for more than one day, then those hormone levels drop down well below the threshold needed to prevent pregnancy,” explains Vanessa Cullins, M.D., vice president for external medical affairs at Planned Parenthood Federation of America.
3. Taking certain medications. There are several fairly uncommon medications that are known to impinge on the Pill’s effectiveness. They include Rifadin, a medication for treating tuberculosis; griseofulvin, a rare anti-fungal drug; some HIV-protease inhibitors; and Tegretol, Dilantin, Mysoline and phenobarbital, which are all anticonvulsants. These medications speed up metabolism in the liver, and hence, the rate at which the Pill’s hormones are broken down. Doctors should be aware of these drugs’ potential impacts on oral contraceptives, but if you suffer from seizures, are HIV-positive or recently contracted tuberculosis or a serious fungal infection, you should speak with your doctor confirm whether or not your medication might fall into this category. Common antibiotics and over-the-counter anti-fungal treatments, on the other hand, do not have an impact on how well the Pill works.