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.

4 Things That Reduce the Pill’s Effectiveness

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.

4. Popping the supplement St. John’s Wort. This herbal dietary supplement can affect the Pill if it is taken regularly and at high doses. Like the medications already mentioned, St. John’s wort speeds up liver metabolism. One study found that women taking 300 milligrams of St. John’s wort three times per day were more likely to experience breakthrough bleeding. Not much more is known about the extent to which the supplement actually poses a threat to oral contraceptive’s effectiveness or at what dose those problems might occur. But to be safe, the National Institutes of Health also recommends using a back-up birth control method for women who are on the Pill and are taking St. John’s wort.

4 Things That Likely Do Not Affect the Pill’s Effectiveness

Now that we’ve covered the culprits that are known to weaken oral contraceptives, it’s worth mentioning the ones that are rumored to affect the Pill’s pregnancy-preventing ability, but either aren’t scientifically verified yet or simply have no effect.

1. Weight. There’s no solid evidence supporting the claim that a woman’s weight affects her birth control’s efficacy. The exception: That does seem to be the case for some overweight women on the patch—perhaps leading to some confusion—and questions still surround weight’s possible effects on over-the-counter emergency contraception. While some websites claim that the hormonal concentrations in oral contraceptives might be “diluted” in overweight women, scientific studies, including one involving 17,000 women conducted in 2001, largely do not support that. One exception is a 2005 study of about 800 women, which found an increased risk of pregnancy in those who were overweight. But those results didn’t stand up in a 2007 study of 1,500 women, which found no significant correlation. The authors of that most recent study, however, did point out that larger studies following women as they do or do not become pregnant would help clarify this question once and for all.

2. Alcohol. No evidence exists that alcohol consumption impacts the Pill’s ability to distribute its hormones throughout the body. If you are an alcoholic and suffer from cirrhosis, then your liver’s impaired function will actually lead to elevated levels of the hormones. So while a dysfunctional liver is a serious problem, cirrhosis likely would not increase your chances of becoming pregnant. What alcohol can do is impede your ability to remember to take the Pill on time, which can leave you vulnerable to an unwanted pregnancy.

3. Taking expired pills. Expired pills that have been kept in a dry, sunlight-free place should still work. Birth control’s expiration date is based on what was proven to work in trials with the Food and Drug Administration, and companies do not test drugs’ effectiveness until those medications ultimately expire. “These steroids are very stable,” Darney says. “So taking expired birth control would be an unlikely cause of pregnancy.”

4. Grapefruit juice. Natural chemicals in grapefruit called furanocoumarin can interact with more than 85 drugs. Birth control pills, however, are not one of them, at least according to the latest scientific literature.If you consistently have trouble remembering to take the Pill, it may be time to look at other birth control options. Planned Parenthood has a quiz that helps determine what method of birth control might be most effective for you. Not all of us have an elephant’s memory for taking the Pill on time, after all.