Related stories about birth control:

• Just like standard birth control, Plan B doesn’t always work. It has a 95% chance of preventing pregnancy if you take it within 24 hours of having unprotected sex.
• Plan B is approved to take up to 120 hours after unprotected sex, but the longer you wait to take it, the less effective it is and the greater your chance of possibly getting pregnant.
• Certain factors can make Plan B less effective, like if you have unprotected sex multiple times in the same cycle or if you are overweight and have a BMI over 35.
• There some side effects to taking Plan B but there are no long-term consequences that will affect your ability to get pregnant in the future.
• This article was reviewed by Rhonia Gordon, MD, who is a clinical assistant professor with the Department of Obstetrics and Gynecology at NYU Langone.
• Visit Insider’s homepage for more stories.

Plan B, sometimes referred to as the “morning-after pill,” is an over-the-counter form of emergency contraception you can take after having unprotected sex. The pill works to prevent pregnancy before it even starts.

However, Plan B doesn’t always work. For your best chance of preventing pregnancy, take it as soon as possible after having unprotected sex, says Nicole M. Bender, MD, an OB-GYN at Keck School of Medicine of USC.

Plan B has a 95% chance of preventing pregnancy if you take it within 24 hours. If you take it within 72 hours, you have up to an 89% chance.

“Plan B is FDA approved for use up to 72 hours after unprotected sex. It can be used up to 120 hours after, but the further out from intercourse, the less effective it will become,” says Bender. So, for max protection, take the pill as quickly as you can get it.

Only one dose is necessary. Taking more than one dose won’t make it any more effective.

How Plan B works

“Plan B consists of a progestin-only hormone called levonorgestrel, which is a synthetic version of progesterone, a naturally occurring hormone in our body,” says Bender. “It’s one pill of 1.5 milligrams of levonorgestrel that you take one time as soon as possible after an act of unprotected intercourse.”

The levonorgestrel prevents or delays ovulation, which is when you release an egg from one of your ovaries. If no egg is released, there’s nothing to be fertilized by sperm, hence no pregnancy.

When Plan B is less effective

One risk factor that can make Plan B less effective is if you have unprotected intercourse repeatedly in the same cycle.

“If Plan B delays ovulation by a few days and then you have another act of unprotected sex a few days later, you can still get pregnant,” Bender says. So, if you want to be safe, be sure to use protection the next time you have sex.

“The other things that can make [Plan B] less effective is having a BMI or body mass index greater than 26,” Bender says. A 2015 study published in Contraception found that as someone’s body weight went up, the effectiveness of levonorgestrel went down.

In fact, the researchers reported that women who weighed 187 pounds, or more, were equally at risk of getting pregnant after unprotected sex whether they used plan B, or not.

Luckily, a more reliable option for overweight people is available. A prescription emergency contraception called Ella, is more effective in women with a BMI that’s up to 35. Ella can be used up to 120 hours after unprotected sex.

If you get pregnant after taking Plan B

Taking Plan B shouldn’t cause your period to be late.

“Usually after taking Plan B, your period either comes a little bit earlier or right on time,” says Bender. “If your period is late at all after taking Plan B, you should really take a pregnancy test.”

If you do end up getting pregnant and decide to continue with the pregnancy, your baby is safe. “We have no evidence that there are any birth defects or any harm to a developing pregnancy,” Bender says. Additionally, using Plan B won’t hurt your future chances of getting pregnant. There are some side effects but no long-term consequences to taking Plan B that affect your fertility.

So if you’ve had unprotected sex and don’t want to get pregnant, take the pill as soon as possible.

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