How Plan B Works Before and After Ovulation

Research Findings vs. FDA Labeling

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.

Plan B Pill box

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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.

What the FDA Labeling Says About Plan B

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. The legal definition of pregnancy "encompasses the period of time from implantation until delivery."

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. In fact, studies have shown that Plan B does not decrease the rate of pregnancy when taken after ovulation. This suggests it may not interfere with fertilization or implantation, just ovulation.

What Research Says About Plan B

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.

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.

The Plan B Implantation Controversy

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:

  • Even if the pill does alter the endometrium, the changes have not been proven to prevent the implantation of a fertilized egg.
  • More importantly, research shows that the single levonorgestrel dose in Plan B doesn’t cause any changes to the lining of the uterus.

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." 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 Plan B Controversy Today

The FDA is has no plans to change the label. 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.

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


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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