by Sarah Kliff

Thanks in part to Plan B's complicated history, there are a lot of myths and misunderstandings about emergency contraception. And despite the fact that the pill has been on the market since 1999, there's stillsome confusion about how it works.

First, don't confuse Plan B,which prevents pregnancy, with RU-486, the pill used in a medical abortion.  “They’re entirely and absolutely different,” says James Trussell, who directs Princeton's Office of Population Research and runs Not-2-Late, a Web site and hotline devoted to emergency contraception.  RU-486contains a synthetic steroid called mifepristone, which interferes with the body’s production of progesterone necessary to sustain pregnancy. Plan B has nothing to do with progesterone. Instead, it inhibits or delays ovulation. Plan B is not effective if the process of implantation has begun.

But even sex-savvy women and men who have that fact down can get confused about the basics of Plan B. That's why we've put together a list of some hard facts about how it works, how it doesn't, and what you should know:

You don't need to take the pills 12 hours apart

Plan B comes with two pills and instructions to take “the second tablet …12 hours after the first dose.” Not a problem if you get the pharmacy at 9 a.m., but a little more inconvenient if your schedule—sexual or otherwise—finds you in need of emergency contraception (EC) after noon. Trussell assures us that you needn't stay up all night or set your alarm for some ungodly hour: Plan B is just as effective when both pills are taken at once.

In fact, the new Plan B One-Step has the same active ingredients of two Plan B pills—1.5mg of a progestin called levonorgestrel—in one tablet. Most EC sold in other countries is a single-pill product. So why have we been advised to space Plan B 12 hours apart? Because it was originally tested as a two-pill product with lower levonorgestrel doses. “Then people said, maybe we could put them all in one, but the danger there is that you might have more side effects,” says Trussell. “But it showed no side effects and the efficacy was the same.”

High doses of your birth control MAY substitute for EC in a pinch

It’s not ideal, but it's doable─if you have the right birth control. Each birth-control brand has different levels of the active ingredients, only some of which can be used as an emergency contraceptive. But there are a decent number that can: flip to page 23 of Trussell’s review article to see if yours is among them. You’ll see that some oral contraceptives can replicate birth control with two pills, but with some it takes six. However, Trussell cautions that studies have show Plan B—which is a progestin-only pill—to be more effective EC than oral contraceptives, which are combined progestin and estrogen pills. So all things considered, he’d still recommend Plan B over an increased dose of oral contraceptives.

There won't be blood

After taking Plan B, some people expect to see spotting, thinking perhaps that the drug might affect the uterine lining and kickstart an early period. This isn't the case, though some women will experience bleeding after taking Plan B. Trussell describes it as “not rare” but also “not common.” The bleeding is called a “withdrawal bleed” and is not menstruation. Rather, it’s caused by the change in hormone levels. It’s more common when Plan B is taken earlier in the cycle (why it’s more common earlier in the cycle, says Trussell, is unknown). (Not-2-Late has more information on bleeding.) Plan B can alter when the next period occurs: it can be a delayed or come early. This is also related to changes in hormone levels.

If you're taking Plan B because you missed several oral contraceptives and don't want to risk it (a smart move) you don’t have to wait for your next period to get back on track. In fact, he makers of Plan B and Plan B One-Step recommend “regular method of contraception be started immediately after.” Trussell adds that, while you can start an oral contraceptive the day after EC, you still need to use condoms or abstain for seven days, since oral contraceptives aren’t effective until you’ve been taking them for a week.

There's more time than you think (but don't delay)

Plan B should definitely be taken as soon as possible after unprotected sex. That’ll increase the odds you’re taking Plan B early enough to inhibit ovulation, thus decreasing the odds of pregnancy. While the pill is indicated for use up to 72 hours after unprotected sex, some studies have shown Plan B to be effective up through 120 hours. Its still better to aim for earlier, but Plan B is not completely ineffective after three days. “There’s no magic difference between 72 and 73 hours,” says Trussell. “The longer you wait the less effective it appears to be. But it’s still much more effective than a prayer.” Amen to that.

EC's Effectiveness Does Not Decrease With Use

The more one takes Plan B after unprotected sex, the more likely it is that you’ll get pregnant─but not because the pill is less effective: the odds of a pregnancy after Plan B are still pretty slim. It reduces the risk of pregnancy by 89 percent. Trussell draws an analogy to the odds of crashing a car: “If you make 1,000 trips, your chances of getting into a car wreck are more than if you take one trip. It’s a cumulative process.” Likewise, the more you use Plan B, the more likely it becomes that you’ll have a failure, even with Plan B’s success rate remaining the same.