How Does Plan B Work?

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.

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.

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.

Summary

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.