Levonorgestrel Emergency Contraception: Plan B

Medically Reviewed by Murtaza Cassoobhoy, MD on March 12, 2023
4 min read

It’s an emergency contraceptive pill taken by mouth after unprotected sex. It’s used to prevent pregnancy. It’s not for routine contraceptive use, and it doesn’t protect you against sexually transmitted diseases, including HIV.

It contains the hormone levonorgestrel. It’s also called the morning-after pill. But you don't have to wait until the morning after sex to take it. In fact, levonorgestrel is more effective the sooner you take it. It is a one-dose regimen: You take one pill. The pill contains 1.5 milligrams of levonorgestrel, which is used in lower doses in many birth control pills.

Levonorgestrel brands include EContra EZ, My Way, Next Choice One Dose, Plan B One-Step, Preventeza, and Take Action. These are given as a one-pill emergency contraception or as two pills taken separately within 72 hours of unprotected sex.

Depending on where you are in your cycle, levonorgestrel helps prevent pregnancy after unprotected intercourse. It may prevent or delay ovulation.

The drug works by stopping the release of an egg from your ovary. It may prevent sperm from fertilizing the egg.

If fertilization does occur, it may prevent the fertilized egg from attaching to the womb. If the fertilized egg is implanted before you take levonorgestrel, the drug won’t work and pregnancy will proceed.

If you take it as directed within 72 hours after you've had unprotected sex, levonorgestrel can reduce your risk of pregnancy by up to 87%. If you take it within 24 hours, it’s even more effective.

But you should know that levonorgestrel isn’t as effective as regular contraception. So don't use it as a form of birth control. And it doesn’t protect you against sexually transmitted diseases. Think of it as a backup – it’s not for routine use. That's why it's called Plan B. 

It can be purchased over the counter at drugstores without a prescription or proof of age. Because it is most effective when taken as soon as possible, consider having a ready supply in your medicine cabinet. Better yet, use a reliable form of birth control and plan for a backup method of birth control.

You can take Plan B One-Step if:

  • You didn't use any birth control.
  • The condom came off or broke.
  • Your diaphragm slipped out of place.
  • You missed at least two or three active birth control pills in a row.
  • You forgot to insert your ring or apply your patch.
  • Your partner didn't pull out in time.
  • You have another reason to think your birth control might not have worked.
  • You were forced to have unprotected sex.

Remember: Plan B will not protect you from getting pregnant if you have unprotected sex after taking it. To protect you against getting pregnant, you need to take it right after you have unprotected sex and use a form of contraception when you have sex.

Don’t take levonorgestrel if:

  • You know you’re pregnant or suspect you might be.
  • You have a history of allergy or are very sensitive to its ingredients.
  • You have a history of recent abnormal vaginal bleeding that your doctor hasn’t yet evaluated.

Many women have taken emergency contraception without serious complications. But it's a good idea to ask your doctor about possible interactions with other medications.

Levonorgestrel is considered safe for most women. You shouldn’t take it if you’re pregnant because it will not end your pregnancy.

Potential side effects of levonorgestrel include:

If you vomit within 2 hours after taking the drug, call a health care professional to find out if you should repeat the dose.

With levonorgestrel, you may also have some unexpected bleeding. It should go away by the time of your next period. 

But it’s possible that levonorgestrel may cause your next period to be heavier or lighter than usual. It may also come earlier or later than is normal for you. If you don't get your period within 3 weeks, get a pregnancy test to make sure you're not pregnant.

Read more on emergency contraception side effects.

No. RU-486, sold as Mifeprex, is a prescription drug for medical abortion. Mifeprex works after you’re already pregnant, while Plan B won't work then. Doctors and the FDA prescribe Plan B as an emergency contraception, not as an abortion pill.