What is plan b

Last updated: April 1, 2026

Quick Answer: Plan B is an emergency contraceptive medication containing levonorgestrel that prevents pregnancy if taken within 72 hours of unprotected intercourse. It works by delaying or preventing ovulation.

Key Facts

What is Emergency Contraception

Plan B is an emergency contraceptive pill designed to prevent pregnancy after unprotected sexual intercourse, contraceptive failure, or sexual assault. It contains levonorgestrel, a synthetic progestogen that works differently than regular birth control. Plan B is available without a prescription in most countries and is intended for backup contraception, not as a primary birth control method.

How Plan B Works

Plan B primarily works by preventing or delaying ovulation, the release of an egg from the ovary. By postponing ovulation, it reduces the likelihood of sperm fertilizing an egg. The medication is most effective when taken as soon as possible after intercourse, particularly within the first 24 hours, though it can work up to 72 hours afterward. The effectiveness decreases significantly with time elapsed since intercourse.

Effectiveness and Usage

Plan B is approximately 60-89% effective at preventing pregnancy depending on timing and individual factors. It's more effective in women with lower body weight. Unlike some emergency contraceptives, Plan B does not interrupt an established pregnancy or cause miscarriage; it only prevents pregnancy from occurring. If a woman is already pregnant, Plan B will not work. Regular contraceptive methods like hormonal birth control, IUDs, or barrier methods are more reliable for ongoing protection.

Availability and Side Effects

Plan B is available over-the-counter at pharmacies without age restrictions in most countries. It can be purchased online and at retailers like CVS and Walmart. Common side effects include nausea, abdominal pain, headache, fatigue, and dizziness. Some users experience changes in their menstrual cycle timing. Serious side effects are rare, and Plan B is generally safe for most women, though it's not recommended as regular contraception due to lower efficacy compared to other birth control options.

Related Questions

How effective is Plan B?

Plan B is 60-89% effective at preventing pregnancy when taken within 72 hours of unprotected intercourse, with highest effectiveness within 24 hours. Effectiveness varies by individual factors and timing, making it less reliable than regular contraception methods.

What are the side effects of Plan B?

Common side effects include nausea, headache, fatigue, abdominal pain, and dizziness. Most side effects are mild and temporary. Some women experience changes in menstrual cycle timing, but serious side effects are rare and Plan B is generally considered safe.

Does Plan B work if you're already pregnant?

Plan B does not work if you're already pregnant and cannot cause a miscarriage or harm an existing pregnancy. It only prevents pregnancy from occurring, so it must be taken before pregnancy is established for it to be effective.

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Sources

  1. Wikipedia - Levonorgestrel CC-BY-SA-4.0
  2. FDA - Plan B Information Public Domain