How Does Plan B Work?

Research Findings vs. FDA Labeling

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Plan B One-Step, often called "Plan B," is an emergency contraceptive (EC). It is also called "the morning-after pill."

Plan B contains a synthetic hormone called levonorgestrel, which can prevent ovulation, or the release of an egg. This is how Plan B works to prevent pregnancy.

Emergency contraceptives like Plan B One-Step prevent pregnancy after birth control failure or unprotected sex. Generic forms of Plan B include My Way, Take Action, and Next Choice One Dose.

Plan B Pill box

Justin Sullivan / Getty Images

This article looks at what the research says about how Plan B works. It also discusses some of the side effects you may experience when taking Plan B.

How Does Plan B Work?

Plan B works by preventing ovulation. It contains a larger dose of levonorgestrel, the synthetic hormone found in birth control pills. When you take this larger dose of levonorgestrel just after unprotected sex, it can stop you from ovulating. Plan B does not work after ovulation.

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. Under "How does Plan B One-Step work?" the label says: “Plan B One-Step may also work ... by preventing attachment (implantation) to the uterus (womb).”

Because of this labeling, people who believe pregnancy begins when an egg is fertilized see Plan B as abortion.

Research does not support this theory about how Plan B works, though.

Studies have shown that Plan B does not decrease the pregnancy rate when taken after ovulation. 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. Because of this, researchers have concluded it cannot prevent implantation of a fertilized egg.

Possible Side Effects

The most common side effect of Plan B is a change in your period. Your period may come sooner or later than expected. You may also have heavier or lighter bleeding or spotting before your period. 

Other side effects of Plan B may include:

  • Tender breasts
  • Cramping
  • Nausea and/or vomiting
  • Headache or dizziness

Plan B is not meant to be used as your primary method of birth control. Though there don't seem to be any health risks or a reduction in effectiveness if you use Plan B frequently, it can cause unwanted side effects. This can include irregular or unpredictable periods.

Plan B is also less effective than long-term birth control, so if you use it often, you should talk to your healthcare provider about other, more effective ways to prevent pregnancy.


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.

Because it is less effective than other types of birth control, Plan B is not meant to be used as a primary form of birth control.

Frequently Asked Questions

  • How long does Plan B take to work?

    It takes about 1.7 hours for the levonorgestrel in Plan B to reach peak levels in your blood. After this, it will delay ovulation for a few days. Plan B is most effective within 72 hours of unprotected sex. 

  • Where can you buy Plan B?

    You can buy Plan B over-the-counter at many major retailers. It can also be found in pharmacies. You do not need a prescription, and you do not need to show ID to purchase Plan B. 

  • How much does Plan B cost?

    Plan B is sold in packs of 1 or 2. Name brand Plan B may cost between $40 and $100, depending on which pack you buy. You can also buy generic Plan B (sold under the names "My Way," "Option 2," etc.), which may cost as little as $8 for one pill.

  • What happens after taking plan B?

    If you haven't already ovulated, Plan B will work to prevent the release of an egg. If you take it after ovulation, it won't work. This is why healthcare providers recommend taking Plan B as soon as possible after unprotected sex.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. 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

  2. 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

  3. Matyanga CM, Dzingirai B. Clinical pharmacology of hormonal emergency contraceptive pills. Int J Reprod Med. 2018;2018. doi:10.1155/2018/2785839

By Dawn Stacey, PhD, LMHC
Dawn Stacey, PhD, LMHC, is a published author, college professor, and mental health consultant with over 15 years of counseling experience.