“Plan B,” also known as the morning after pill, is a brand name for a type of emergency contraception that comes in the form of a pill. The World Health Organization (WHO) states that emergency contraception can be effective at preventing pregnancy up to 95% of the time when taken within five days following sexual activity.

Emergency contraception should be taken within 72 hours to prevent unwanted pregnancy in the instances of:

• Unprotected sex,
• Potential contraceptive failure (condom breaking, or missed regular birth control pill),
• Incorrect use of contraceptives,
• And in the instance of sexual assault without contraceptive use.

How Does it Work?

The most common form of emergency contraception is called levonorgestrel. If you take regular hormonal birth control, you may recognize this medication. Taken at a much higher dose (1.5 mg vs. 0.1 mg), this works by delaying or preventing ovulation, which is the event where women pass an egg through their fallopian tube and into the uterus to be fertilized by sperm. Additionally, it may help thicken the mucus at the cervix, which can make it harder for sperm to enter the uterus.

Other combination emergency contraceptives exist, however levonorgestrel is the most common.

In order to be most effective, it’s important to take an emergency contraceptive pill as soon as possible after unprotected sex, contraceptive failure, etc. The longer the wait, the higher the chance of pregnancy. Additionally, emergency contraception will not work if ovulation has already happened. The egg has already been released at this point, meaning levonorgestrel can no longer work its magic.

The morning after pill is not a method of abortion. Meaning, it is not necessary or recommended to take any form of emergency contraception while pregnant, despite causing no reported harm to mother or fetus. Emergency contraception is not recommended for those who are breastfeeding.

Are There Any Risks?

Generally, there are no adverse effects of the morning after pill outside of those associated with everyday hormonal birth control, such as:

• Headache
• Fatigue
• Tender breasts
• Pain or cramps (similar to a period)
• Light bleeding between periods, heavier menstrual bleeding, etc.
• Dizziness
• Nausea or vomiting (note: if you vomit within three hours of taking the pill, then it’s necessary to take another dose)

Additionally, it’s important to note that the morning after pill can affect the menstrual cycle. Women can experience delayed periods, “skipped” periods, or infrequent periods after taking it. Do not take more than what is recommended to avoid messing with natural hormonal processes and to avoid more stress from late or missed periods.

Key Takeaways

• “Plan B” is a brand name for the emergency contraceptive levonorgestrel, which can be taken to avoid unwanted pregnancy after unprotected sex (amongst other combinations).
• Emergency contraception is generally safe, with minimal side effects, but may delay or stop the normal period cycle a woman experiences.
• Emergency contraception does not work effectively as an abortion medication. It is important to avoid this medication with an established pregnancy and while breastfeeding.
• Emergency contraception is available over the counter or by prescription – speak with your healthcare provider about the regulations and availability in your area.

Resources

• Adashi, E. Y., Cohen, I. G., & Wilcox, A. J. (2023). The FDA declares levonorgestrel a nonabortifacient—a 50-year saga takes a decisive turn. JAMA Health Forum, 4(8). https://doi.org/10.1001/jamahealthforum.2023.2257
• Patel P, Bajaj T. Levonorgestrel. [Updated 2025 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539737/
• World Health Organization. (2021, November 9). Emergency contraception. World Health Organization. https://www.who.int/news-room/fact-sheets/detail/emergency-contraception