Emergency contraception. Plan B. The morning after pill. You know what it’s for, but do you know all the facts—like how it works, how effective it is, and what to do if you ever need it? There are a ton of stereotypes and misconceptions, but when your plan A fails (or, well, doesn’t show up at all), you’re going to want to know the truth. Here, we’ve debunked nine common myths about the morning after pill:

Myth: You’ll feel like crap after you take it

Scared of side effects? Experts say they’re rare and typically mild. "In the old days when we used to use high doses of estrogen, people would get queasy and have sore breasts,” says ob-gyn Mary Jane Minkin, M.D., clinical professor at Yale University Medical School. "But this is no estrogen at all. This is just progestin.” While you may feel some nausea or headaches after taking it, this will typically go away after a day or two. The more common side effect Minkin notes is a wacky period that month, which may be a little longer or shorter than usual.

Myth: It can stop pregnancy once it’s started

One major misconception about the morning after pill is whether or not it can halt a pregnancy. Research shows that the main mechanism of action is inhibiting or delaying ovulation—so the egg wouldn’t even be released, says Minkin. Some research has looked at the possibility that emergency contraception might also affect the lining of the uterus in a way that would make implantation harder (on the off chance that an egg was already released and fertilized, but not implanted, when you took the pill) but other studies have not found evidence for this. So what if the egg was already released, fertilized, and implanted before you got a chance to take the pill? In that case, you’re pregnant, and Plan B won’t change that.

Myth: It’s only effective if you take it right away

It’s been over 72 hours and you still haven’t taken it—you’ve missed your window, right? Wrong. While it’s true that you should take it as soon as possible after unprotected sex, recent studies have found that it’s still effective up to four days after intercourse.

Myth: It’s only for people who had unprotected sex

False. Maybe the condom broke or it slipped off inside you. Maybe you’re taking birth control, but you forgot a few (okay, four) pills this month. Birth control failures happen, and that’s just as good a reason to use emergency contraception as having unprotected sex. Minkin advises that it’s still perfectly safe to take the morning after pill even if you’re currently taking birth control but forgot a few doses.

Myth: It’s not as effective in heavier women

You may have heard last year that Norlevo, the European version of Plan B, added a warning that the drug was less effective in women over 165 pounds and ineffective for women over 175 pounds. Luckily, the European Medicines Agency recently reviewed the research and announced that there is not sufficient data to support this.

Myth: There may be long-term effects of using it

The morning after pill may seem like a relatively new drug, but the main ingredient in it—levonorgestrel—is the same form of progestin found in other forms of birth control, like the IUD. "It’s extremely safe,” says Trussell. "Levonorgestrel has been around for a long time.”

Myth: It’s a reliable form of birth control

"There are no safety concerns with using it every week; however, it’s not as effective as any ongoing method of contraception,” says Trussell. Emergency contraception is still highly effective—according to drug makers, about seven out of every eight women who would have gotten pregnant will not become pregnant when they take it after unprotected sex—but it is still not as reliable or effective as routine birth control.

Agencies