How Does Plan B Work?


Plan B, also known as “the morning after pill” or levonorgestrel, is a medication that is available over the counter to women who want to prevent pregnancy after an episode of unprotected sex. Marketing for the drug makes wild claims – including that Plan B is 95% effective at preventing pregnancy after unprotected sex.

So what is Plan B?

Levonorgestrel is a synthetic (non-natural) progesterone, which is a hormone present in every woman’s reproductive cycle. It is a component of many forms of birth control as well as the popular hormonal IUD, the Mirena. 

How does it actually work?

Understanding this requires a little more in-depth knowledge of the reproductive cycle. Before a woman can become pregnant, her brain releases a hormone called LH (luteinizing hormone). This happens only once a month. This LH surge results in the release of an egg. The egg only lives about 24 hours. If sperm are around to fertilize the egg, fertilization can occur, and the fertilized egg (an embryo) will then implant into the uterus and continue to grow. Sperm can actually live up to 5 days, so if sex occurs 5 days before the egg is released, the woman can still become pregnant.

So where in this process does Plan B come in?

Mostly, Plan B is going to work by delaying ovulation. The high dose of progesterone will cause a feedback loop and block the brain’s release of LH, thereby delaying ovulation. Other ways Plan B works is by thickening cervical mucus so that sperm cannot enter the uterus. Studies have not shown that Plan B prevents implantation. This is still debated, because levonorgestrel as a medication does cause multiple changes at the level of the endometrium in the form of OCPs and Mirena, but probably not at the Plan B dosage. 

What are the physical side effects?

Nausea and vomiting, heavier bleeding with your period, abdominal pain, dizziness, headache, breast tenderness. Roughly 20% of women will experience some or all of these side effects.

How effective is Plan B at preventing pregnancy?

Not very. To review, the only way it is really effective is if taken before the LH surge. So there is really only a 4-5 day window where Plan B could be effective. For 2/3 of the cycle, a woman is not even fertile. Plan B is not effective if ovulation or the LH surge has already occurred. Women are spending money on this drug (which costs $40-50) and exposing themselves to the side effects when they may not have become pregnant at all or they are not in this very narrow window when the drug may be effective. Additionally, Plan B may not work at all for women over 165 lbs. Considering that the average women in America is 166 lbs, the pharmaceutical company that makes Plan B is likely making a lot of money off women in whom this will never be effective. The 95% success rate touted by Plan B is a myth.

If you think you might be pregnant (even if you have taken Plan B), schedule an appointment with Birth Choice to confirm your pregnancy and discuss your options. Schedule online or call us at 214-631-2402. 



  • Durand et al. On the Mechanisms of action of short term levonorgestrel administration in emergency contraception. Contraception. Vol 64, issue 4, October 2001, pages 227-234
  • Contraception. 2007 Jun;75(6 Suppl):S93-8. Epub 2007 Mar 23.
  • ACOG Practice Bulletin No. 152: Emergency Contraception. Obstet Gynecol. 2015;126(3):e1-e11. doi: 10.1097/01.AOG.0000471180.02939.1a.[PubMed 26287787
  • Plan B (levonorgestrel) [prescribing information]. North Wales, PA: Teva Pharmaceuticals USA Inc; September 2017
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Saturday, 27 November 2021