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How Plan B Works

Research Findings vs. FDA Labeling

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Updated March 07, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

How Plan B Works

How Plan B & Plan B One-Step Work

Photo © 2012 Dawn Stacey

Many women may choose to use Plan B One-Step (its generic forms, My Way and  Next Choice One Dose) or the generic version of Plan B (Next Choice), as emergency contraception to prevent pregnancy after birth control failure or unprotected intercourse. Even though emergency contraception is safe and effective, the use of Plan B continues to generate controversy. Much of this debate stems from how Plan B works--specifically with respect to whether or not Plan B prevents fertilized eggs from implantation. It also seems that a major factor contributing to the confusion about how Plan B works is that the FDA labeling for this product says one thing, yet research and data does not confirm the FDA’s labels.

What the FDA Labeling Says

According to product labels, Plan B works mainly by preventing/delaying ovulation or by avoiding fertilization (by altering the movement of sperm and/or egg). The debate arises from the next part of the label which, under "mechanism of action," reads:

“In addition, it [Plan B] may inhibit implantation (by altering the endometrium).”

 

The medical community defines pregnancy as being established once a fertilized egg has implanted into the uterus. Yet those who hold personal viewpoints that pregnancy starts at fertilization believe that Plan B is akin to abortion since its use can prevent implantation of a fertilized egg. That being said, research does not support this theory about how Plan B works; in fact, studies show that when taken after ovulation, Plan B does not decrease the rate of pregnancy.

What Research Shows

  • Data from recent studies continues to provide strong evidence that Plan B does not prevent implantation (this also seems to be true for Ella, another emergency contraceptive option). The majority of research claims that the primary way that Plan B works is by preventing or delaying ovulation--if Plan B is taken before ovulation occurs. By not allowing the egg to release, there is nothing available for sperm to fertilize, thus preventing a pregnancy from occurring.

  • Research also suggests that the progestin (levonorgestrel) in Plan B may help avoid pregnancy by altering the movement of sperm. Some studies report that this progestin can cause cervical mucus to thicken, making it harder for sperm to reach an egg. Additional data reports that the presence of this progestin can cause sperm to become hyperactive or “confused,” thereby affecting the direction in which they swim.

  • In several research studies, women were given Plan B after determining (via hormone tests) which women had ovulated and which had not. The results all showed that none of the women who took Plan B before ovulation became pregnant (these results really help to emphasize how Plan B works by delaying ovulation). Pregnancies only occurred in women who took Plan B on or after the day of ovulation, and these women became pregnant at the same rate as women who have not taken Plan B. Such results prompted researchers Noé, Croxatto, Salvatierra, and Reyes (2011) to conclude that Plan B
    “Does not prevent embryo implantation and therefore cannot be labeled as abortifacient.”
  • In 2007, studies demonstrated evidence that Plan B did not prevent the attachment of human embryos to the uterine wall. Researchers obtained discarded fertilized eggs from fertility clinics. After exposing these eggs to the progestin (found in Plan B) in a simulated environment, results demonstrated that the presence of the progestin did not prevent the eggs from attaching to cells that line the uterus.

So, the majority of research reveals that using Plan B does not cause any changes in the lining of the uterus (endometrium). Because Plan B does not have any effect on endometrium, researchers have concluded that this emergency contraceptive cannot prevent implantation of a fertilized egg. Many researchers have further explained  the notion that Plan B does not stop implantation from occurring is probably the reason that it's not 100% effective at preventing an unintended pregnancy--as well as why Plan B is less effective the longer you wait to use it.

Why the FDA Included Implantation on Plan B Labels:

From the very beginning of Plan B’s approval process, its manufacturer requested that the FDA not include implantation as one of the ways Plan B works on its label. It appears, though, that during the approval process, most of the research on Plan B investigated whether its active ingredient, the progestin levonorgestrel, could safely and effectively prevent pregnancy. These studies did not explore how Plan B works. So, the FDA may have decided to include implantation on Plan B’s labeling because this mechanism seems to be one of the ways that birth control pills (those that contain levonorgestrel as well as brands that use a different progestin) work--by changing the lining of the uterus. So the FDA may have decided that because the pill may do this, so can Plan B.

But there are two things to keep in mind. One, even if the pill does alter the endometrium, the changes it causes have not been proven to hamper the implantation of a fertilized egg. Yet more importantly, with birth control pills, you are building up progestin doses because you take these pills every day. Research shows that the single levonorgestrel dose in Plan B doesn’t have the strength or time to cause any changes to the lining of the uterus.

Yet those involved in the Plan B approval process report that the FDA decided to include, on the product label, the possibility that Plan B works "theoretically ... by interfering with a number of physiological processes." The FDA then expanded upon this statement by including 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 mechanism of action was still included on the drug’s label.

How Plan B Works - Conclusion

Though it is unknown whether the FDA is considering whether or not to allow for label revision, Erica Jefferson, an F.D.A. spokeswoman, acknowledged, "The emerging data on Plan B suggest that it does not inhibit implantation." Research does make clear that Plan B will not interrupt an existing pregnancy, and works primarily by preventing ovulation. Because the research overwhelmingly shows that Plan B does not prevent implantation of a fertilized egg, the research community (and many in the medical community) feel that such language about implantation should be removed from Plan B labeling.

 

Help in Navigating Some of the Confusion Around Plan B

 

Sources

 

Duramed Pharmaceuticals. (2009). Plan B One-Step: HIGHLIGHTS OF PRESCRIBING INFORMATION. Accessed 9/10/212

Durand M, del Carmen Cravioto M, Raymond EG, Duran-Sanchez O, De la Luz Cruz-Hinojosa M, Castell-Rodriguez A, Schiavon R, Larrea F. On the mechanisms of action of short-term levonorgestrel administration in emergency contraception. Contraception 2001; 64(4): 227-234. Accessed via private subscription.

Lalitkumar PG, Lalitkumar S, Meng CX, Stavreus-Evers A, Hambiliki F, Bentin-Ley U, & Gemzell-Danielsson K. Mifepristone, but not levonorgestrel, inhibits human blastocyst attachment to an in vitro endometrial three-dimensional cell culture model. Human Reproduction 2007; 22(11): 3031-3037. Accessed via private subscription.

Marions L, Hultenby K, Lindell I, Sun X, Stabi B, & Gemzell-Danielsson K. Emergency contraception with mifepristone and levonorgestrel: Mechanism of action. Obstetrics and Gynecology 2002; 100(1): 65-71. Accessed via private subscription.

Meng CX, Marions L, Bystrom B, & Gemzell-Danielsson K. Effects of oral and vaginal administration of levonorgestrel emergency contraception on markers of endometrial receptivity Human Reproduction 2010; 25(4): 874-883. Accessed via private subscription.

Noé G, Croxatto HB, Salvatierra AM, & Reyes R. Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation. Contraception 2011; 84(5):486-492. Accessed via private subscription.

Prine L. Emergency contraception: Myths and facts. Obstet Gynecol Clin N Am.. 2007; 34:127–136. Accessed via private subscription.

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