If you've heard that antibiotics can reduce the effectiveness of oral contraceptives, you're only partially right. Common antibiotics like penicillin and amoxicillin aren't going to do anything to your birth control, contrary to popular belief. But a kind of antibiotic used to treat TB, called rifampicin, has been shown to have adverse reactions with the combined contraceptive pill, the progesterone-only pill, and the progesterone-only implant. (It doesn't seem to have any effect on IUDs or injections, and if you're using the levonorgestrel emergency contraceptive, AKA Plan B, you just need to take a double dose.)

The reason this antibiotic messes with your birth control is that rifampicin-type antibiotics are, unlike other types, enzyme inducers. Like other medications that don't react well with contraception, this kind of medication works in the body to increase enzyme levels, which help to break down hormones — like birth control hormones. More enzymes means lower levels of birth control hormones in your system, and in turn, a lowered effectiveness rate.

We should all have learned this in high school, but because not everyone has access to comprehensive sex ed, it apparently still needs to be said: condoms are never foolproof, even if they don't break. (Think of Girls' Hannah Horvath's fear of "the stuff that gets up around the sides of condoms.) That being said, there's an easy way to reduce the risk of a condom's breaking. Latex condoms are vulnerable to degrading if you use lubrication that's not water- or silicone-based. Oil-based lubricants erode the surface of latex and risk breakage, so always make sure you have a condom-safe lube (or a lube-safe condom) before combining the two.

Some women manage their depression with the herb St. John's Wort over pharmaceutical antidepressants, even though it hasn't been proven as effective. The herb could also inhibit the effectiveness of hormonal contraception because it's an enzyme inducer, which as we learned earlier, speeds up the breakdown of hormones, making them less effective than usual. The effectiveness of contraception may also be altered by SSRIs, because they can shift the levels of hormones in women's bloodstreams and exacerbate side effects of birth control, or even potentially reduce their usefulness. No matter how you choose to manage your mood disorder, you should always check with your doctor about possible interactions of any new treatment.

Anti-seizure medication comes in two forms, and women who have epilepsy should take special note of which kind their medicine falls under. One group of medications is an enzyme inducer, which, you guessed it, the former can interfere with hormonal contraception. One kind of non-enzyme inducing medication, however, can have its effectiveness reduced by birth control, not the other way around. Lamotrigine has been occasionally shown to be affected by the Pill, and though it's unclear if that interference goes the other way, it's always important to double and triple check what interactions your medications may have.

This sounds like an urban myth, but it isn't — depending on volume and timing. Drinking grapefruit juice may, in theory, interfere with how the hormones in oral contraception are absorbed because it increase sthe levels of enzyme CYP3A4, which is one of the main ways the body metabolizes oral contraceptives. However, experts don't advise a grapefruit-free diet. They just propose that you don't exist purely on this delicious fruit alone, and that you take your birth control at a different time of day than your normally-scheduled grapefruit snack. People on a grapefruit "cleanse," though, should beware: apart from severe nutrition issues stemming from eating too much of one thing, you could be headed into pregnancy trouble, too.

If you take your birth control orally, then find yourself vomiting (or experiencing diarrhea) before the pill has time to absorb through your system, it may not be as effective as advertised. (If you have unprotected sex during this period, it's a good idea to take emergency contraception.) Chronic bowel issues, from irritable bowel syndrome to Crohn's Disease, can also impede the absorption of contraceptives, as does any other digestive issue. In fact, the synthetic progestin drospirenone, which is found in the combined pill, is actually known to make irritable bowel syndrome worse. If you've got bowel or digestion issues, you're better off having contraceptives that digestion won't interfere with, like IUDs or injections.

Women who are being treated with HIV have a range of treatment options, but several of them — ritonavir, ritonavir-boosted protease inhibitors, lopinavir, efavirenz, and nevirapine — have been linked to problems with the combined pill and the contraceptive implant. Women on efavirenz who have the implant, for instance, have been known to experience "contraceptive failure" (i.e., get pregnant). The treatments in question are, once again, enzyme inducers, but their effects are all over the place: they can either reduce effectiveness or increase hormonal side effects by playing havoc with the body's metabolizing of hormones. Another kind of HIV medication, marketed as Stribild, may have negative effects as well, though it's not as pronounced as the others. HIV specialists tend to recommend that people taking Stribild take a higher dose of contraception to offset any possible effectiveness issues.