How Long Can Plan B Work

How Long Can Plan B Work

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How effective is Plan B and how late can you take it?

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Medically reviewed by Sally Chao, MD . Last updated on July 30, 2021.
Plan B ( levonorgestrel ) is estimated to be 60% to 94% effective at preventing pregnancy when taken within 3 days (72 hours) after unprotected sex. Somewhere between 0.6% to 2.6% of women who take Plan B within this time frame will still become pregnant.
While Plan B can be taken up to 72 hours after unprotected sex, its efficacy decreases with time, and it is best to take it as soon as possible. For example, the chances of pregnancy are higher if Plan B is taken 4 days after unprotected sex, but are subsequently lowered if taken 3 days, 2 days, 1 day or within 24 hours after unprotected sex. However, the American College of Obstetricians and Gynecologists notes that some studies suggest that it can be moderately effective at preventing pregnancy up to 5 days after unprotected sex. Five days after unprotected sex is the latest time with which Plan B has been shown to lower your chances of pregnancy.
The efficacy of Plan B is also affected by the time at which it is taken within your menstrual or reproductive cycle. Plan B works best when taken before ovulation, or when an ovary releases an egg. Once there is a surge in the hormone that initiates ovulation and ovulation begins, Plan B cannot prevent pregnancy. Its primary mechanism of action is to change hormone levels such that ovulation is delayed. This is why it is important to take Plan B as soon as possible after unprotected sex. As treatment is delayed, ovulation becomes more imminent (if it has not yet occurred).
A review of studies in Clinical Obstetrics and Gynecology showed that among 492 women who took Plan B after unprotected sex, those who took it on or after the day of ovulation had the same pregnancy rate as would be expected among women who used no contraception at all. By comparison, none of the women who took Plan B before ovulation became pregnant.
In the event of another unprotected sexual encounter after Plan B is taken, Plan B will not protect against pregnancy—you will need to take Plan B again after another unprotected sexual encounter.
Plan B may also be less effective at preventing pregnancy in women who are overweight or obese. Studies reviewed by Cochrane suggest that the dose of levonorgestrel in Plan B has a lowered potency when taken by women who have a higher body mass index.
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Drugs.com provides accurate and independent information on more than 24,000 prescription drugs, over-the-counter medicines and natural products. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment. Data sources include IBM Watson Micromedex (updated 5 July 2022), Cerner Multum™ (updated 27 June 2022), ASHP (updated 16 May 2022) and others.
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How Plan B Works Before and After Ovulation

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Verywell Health's content is for informational and educational purposes only. Our website is not intended to be a substitute for professional medical advice, diagnosis, or treatment.

Ⓒ 2022 Dotdash Media, Inc. — All rights reserved

Verywell Health is part of the Dotdash Meredith publishing family.

Monique Rainford, MD, is board-certified in obstetrics-gynecology, and currently serves as an Assistant Clinical Professor at Yale Medicine. She is the former chief of obstetrics-gynecology at Yale Health.

Plan B One-Step is an emergency contraceptive. Emergency contraceptives are used to prevent pregnancy after birth control failure or unprotected sex. Generic forms of Plan B include My Way , Take Action , and Next Choice One Dose .

Emergency contraception is safe and effective. Still, Plan B is controversial . Much of the debate is based on a misunderstanding about how Plan B works.

Some people believe Plan B prevents a fertilized egg from implanting in the lining of the uterus. This belief is based on the product's FDA labeling. Research, however, has not confirmed the information on the FDA label.

This article will discuss the controversy around the FDA labeling for Plan B. It will also look at what research says about how Plan B actually works.

According to the product label, Plan B prevents or delays ovulation, or the release of an egg. It may also prevent fertilization, when an egg and sperm combine.

The debate is based on another statement made on the label. Under "How does Plan B One-Step work?" the label says: “It is possible that Plan B One-Step may also work... by preventing attachment (implantation) to the uterus (womb).”

The medical community does not agree on the definition of pregnancy. 1 The legal definition of pregnancy "encompasses the period of time from implantation until delivery." 2

Some people, though, believe pregnancy begins when an egg is fertilized. People who hold this belief see Plan B as a form of abortion . This is because it is said to prevent implantation of a fertilized egg.

Research does not support this theory about how Plan B works, though. 3 In fact, studies have shown that Plan B does not decrease the rate of pregnancy when taken after ovulation. 4 This suggests it may not interfere with fertilization or implantation, just ovulation.

Most research suggests that Plan B does not cause changes in the endometrium , or lining of the uterus. 3 Because of this, researchers have concluded it cannot prevent implantation of a fertilized egg.

Plan B is not 100% effective at preventing an unintended pregnancy. It is also less effective the longer you wait to use it. Many researchers think this is because it does not prevent implantation.

Research shows Plan B does not cause any changes in the lining of the uterus. Therefore, it would not prevent implantation of a fertilized egg.

During Plan B's approval process, its manufacturer asked the FDA to remove the implantation piece from its label. It is not completely clear why the FDA chose to keep it there.

Most of the research done during the approval process focused on Plan B's active ingredient, the progestin hormone levonorgestrel . The research looked at the hormone's safety and its effectiveness at preventing pregnancy. These studies did not explore how Plan B works.

The FDA may have decided to include implantation on Plan B’s label because this is one of the ways birth control pills work . Birth control pills containing levonorgestrel or a different progestin change the lining of the uterus. The FDA may have decided that because the pill may do this, Plan B might, too.

But there are two things to keep in mind:

According to those involved in the Plan B approval process, the FDA thought it was important to include on the label the possibility that Plan B works "theoretically ... by interfering with a number of physiological processes." 6 This is why it included a list of all the possible ways that Plan B works.

Even though there was no scientific proof that Plan B works by preventing eggs from implanting, this claim was still included by the FDA on the drug’s label.

The FDA is has no plans to change the label. 7 But Erica Jefferson, an FDA spokesperson, did tell the New York Times that "the emerging data on Plan B suggest that it does not inhibit implantation." Research is clear that Plan B does not interfere with implantation. It works primarily by preventing ovulation. 8

Because of this, the research community and many in the medical field feel the language about implantation should be removed from Plan B labeling. 7

The FDA labeling for the Plan B emergency contraceptive says Plan B may work by preventing implantation of the fertilized egg. This has created controversy over its use. Research, however, shows that Plan B does not work this way. Instead, it works by preventing ovulation and fertilization of the egg.

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Chung GS, Lawrence RE, Rasinski KA, Yoon JD, Curlin FA. Obstetrician-gynecologists' beliefs about when pregnancy begins . Am J Obstet Gynecol. 2012;206(2):132.e1-7. doi:10.1016/j.ajog.2011.10.877
Gemzell-Danielsson K, Berger C, Lalitkumar PGL. Emergency contraception — mechanisms of action . Contraception . 2013;87(3):300-308. doi:10.1016/j.contraception.2012.08.021
Novikovaa N, Weisberg E, Stanczyk FZ, Croxattod HB, Fraser IS. Effectiveness of levonorgestrel emergency contraception given before or after ovulation — a pilot study . Contraception . 2007;75:112-118. doi:10.1016/j.contraception.2006.08.015
Gemzell-Danielsson K, Berger C, Lalitkumar PGL. Emergency contraception — mechanisms of action . Contraception . 2013;87(3):300-308. doi:10.1016/j.contraception.2012.08.021
There was an error. Please try again.

Medically Reviewed by Brunilda Nazario, MD on April 17, 2020
Levonorgestrel is a hormone that can be used for emergency contraception. Emergency contraception should not be used as a routine method of birth control.
Levonorgestrel can prevent pregnancy after unprotected sex. People sometimes call it 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 2 pills taken separately within 72 hours of unprotected sex.
Depending upon where you are in your cycle, levonorgestrel helps prevent pregnancy after unprotected intercourse. It may prevent or delay ovulation .
Levonorgestrel is not the same as RU-486, which is an abortion pill. It does not cause a miscarriage or abortion. It will not work if you are already pregnant when you take it. Learn more about how emergency contraception works .
If you take the pill within 72 hours after you've had unprotected sex, levonorgestrel can reduce the risk of pregnancy by up to 87% if taken as directed. If you take Plan B One-Step within 24 hours, it is much more effective.
But you should know that Plan B One-Step is not as effective as regular contraception . So don't take it as a form of birth control . And, it does not protect you against sexually transmitted diseases. Think of it as a backup -- not for routine use. That's why it's called Plan B. Get more information on emergency contraception and its effectiveness.
Plan B One Step 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 (up to 72 hours after unprotected sex), 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.
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 a contraception when you have sex
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 should not take it if you are pregnant because it will not end the pregnancy.
Potential side effects of levonorgestrel include:
If you vomit within two hours after taking the drug, call a healthcare 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. However, it is 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 three weeks, get a pregnancy test to make sure you're not pregnant. Read more on emergency contraception side effects .
FDA: "FDA's Decision Regarding Plan B: Questions and Answers" and "Plan B (0.75mg levonorgestrel) and Plan B One-Step (1.5 mg levonorgestrel) Tablets Information."
The National Women's Health Information Center: "Emergency Contraception (Emergency Birth Control)."
Planned Parenthood: "Emergency Contraception (Morning After Pill)."
Office of Population Research, Princeton University: "Get Emergency Contraception NOW."
University of Illinois at Urbana-Champaign: "Plan B One-Step Emergency Contraception."
UpToDate.com: "Emergency Contraception." "Intrauterine Contraception."
What you should know to prevent pregnancy.
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