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Morning after pill: how does emergency contraception work?

felix gussone

Reviewed by Felix Gussone, MD, written by Patricia Weiser, PharmD

Last updated: Jan 25, 2022
5 min read


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

If you currently don’t want to get pregnant, you’re likely using some form of birth control. Maybe it’s condoms. Maybe it’s the pill. Whatever your choice of contraceptive is, it’s important to understand that it is not always 100% effective. Condoms break, and you might forget to take your birth control pill. So, what do you do if you realize your choice of birth control has failed? One option is to turn to emergency contraception (EC), more commonly known as the morning after pill, to help reduce your chances of an unwanted pregnancy. 

There are two types of morning after pills— levonorgestrel (Plan B) and ulipristal (Ella)—that you can take following unprotected sex. Let’s learn a bit more about how they work and when they’re most effective. 


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What is the morning after pill?

The “morning after pill” refers to a type of birth control called emergency contraception (EC). Unlike other birth control methods used before sex, women can take EC after unprotected intercourse to help reduce the chance of becoming pregnant. 

EC is meant to be used if your usual birth control method failed, wasn’t used, or wasn’t used correctly. For example, if a condom broke or wasn’t worn the entire time, your birth control patch came off, or you missed a dose of your birth control pills (Turok, 2021; FDA, 2019; FDA, 2021).

Despite being called the “morning after pill,” an emergency contraceptive pill does not necessarily have to be taken in the morning. But, it should be taken as soon as possible after unprotected sex, within 3 to 5 days at the latest.

Do I need a prescription for the morning after pill?

Whether or not you need a prescription for EC depends on the type of pill. There are two main types of EC pills available: levonorgestrel (Plan B), which is sold over the counter (OTC), and ulipristal (Ella), which requires a prescription from a healthcare provider. You do not need to wait until you have an emergency to get EC; you can get it and keep it on hand, just in case.

How much does the morning after pill cost?

The retail price of the morning after pill ranges from $41 to $50; generic versions cost a bit less (GoodRx, n.d.). Since Ella is a prescription-only drug, many insurance plans will cover it. And even though an Rx isn’t required for Plan B, some medical assistance and insurance plans will cover it with a healthcare provider’s prescription. 

You may also be able to pay for EC using an FSA or HSA, but check with your plan for details.

How does the morning after pill work?

Levonorgestrel and ulipristal reduce the chance that sperm will come in contact with and fertilize an egg. EC works to help prevent pregnancy by temporarily stopping or delaying your ovaries from releasing an egg, known as ovulation.

Depending on where you are in your menstrual cycle when you have unprotected sex, it may not be possible for EC to delay ovulation. But it can still help reduce the chance of sperm fertilizing an egg. EC is also thought to help prevent a fertilized egg from implanting into the uterus lining (FDA, 2015; FDA, 2021).

Taking either levonorgestrel or ulipristal does not interfere with a fertilized egg that has embedded itself in the uterus lining. In other words, EC won’t do anything if you’re already pregnant. It is not an abortion pill; it cannot reverse a pregnancy or harm an unborn baby (Turok, 2021; WHO, 2021).

Keep in mind that one dose of levonorgestrel or ulipristal is meant to reduce the chance of pregnancy after one encounter of unprotected intercourse. It does not keep working to prevent pregnancy with subsequent encounters of unprotected sex. Also, the morning after pill does not protect against sexually transmitted infections (STIs) (WHO, 2021).

How effective is the morning after pill?

The morning after pill is meant to reduce the chance of getting pregnant, but it’s not 100% effective. To lower your risk of pregnancy, you’ll want to take EC as soon as possible after unprotected sex or birth control failure, within 3 to 5 days.

If you’re wondering which morning after pill is better, clinical trials have shown that ulipristal is more effective and works for a longer time window than levonorgestrel (Glasier, 2010). Ulipristal works to reduce the risk of pregnancy for up to 120 hours (5 days) after sex, and levonorgestrel works best within 72 hours (3 days) after sex. Ulipristal gives you a longer time window, but both options are more effective the sooner you take them.

The chart below summarizes some key information about each one (Turok, 2021; FDA, 2019; FDA, 2021): 

What makes the morning after pill less effective?

A few things can affect how well EC works to prevent pregnancy. It may not work as well if you take certain prescription drugs or dietary supplements, such as seizure medications or the herb St. John’s Wort (FDA, 2009; FDA, 2021). 

Also, if you’re already using hormonal birth control (like the pill, patch, or vaginal ring), taking ulipristal (Ella) can make both drugs less effective, so doctors typically recommend waiting 5 days after taking ulipristal to restart your usual hormonal birth control. For this reason, you should use condoms or another barrier birth control method until your next period after taking ulipristal (FDA, 2021). 

Additionally, levonorgestrel (Plan B) may be slightly less effective in women with obesity, defined as having a body mass index (BMI) greater than 30 (Festin, 2017). According to Planned Parenthood, if you weigh 155 pounds or more, Ella is a better choice than Plan B. But if you weigh more than 195 pounds and need emergency contraception, talk to a healthcare provider about other options (Butler, 2020).

Side effects of the morning after pill

The morning after pill is safe and doesn’t cause long-term side effects. If you do experience any side effects, they should be temporary and mild, such as (FDA, 2019; FDA, 2021):

  • Changes in your menstrual cycle, like an early or late period, or lighter or heavier-than-normal bleeding
  • Dizziness
  • Headache
  • Nausea
  • Sore breasts
  • Vomiting
  • Pain in your lower abdomen

When to contact a healthcare provider

If you vomit within 2 to 3 hours after taking a morning after pill, contact a healthcare provider to check whether you should take another dose.

And if you start having severe abdominal pain after taking EC, especially if it’s a week or two later, you should seek immediate medical attention, because severe abdominal pain can be a sign of an ectopic pregnancy, which is a medical emergency.

Finally, if you have questions about taking the morning after pill or are curious about various first-line forms of birth control (like pills, rings, and patches), reach out to a medical professional. They can offer you personal medical advice, including more details about which type of EC would work best for you.


  1. Festin, M., Peregoudov, A., Seuc, A., Kiarie, J., & Temmerman, M. (2017). Effect of BMI and body weight on pregnancy rates with LNG as emergency contraception: analysis of four WHO HRP studies. Contraception, 95(1), 50–54. doi: 10.1016/j.contraception.2016.08.001 Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5357708/ 
  2. Glasier, A. F., Cameron, S. T., Fine, P. M., Logan, S. J., Casale, W., Van Horn, J., et al. (2010). Ulipristal acetate versus levonorgestrel for emergency contraception: a randomised non-inferiority trial and meta-analysis. Lancet, 375(9714), 555–562. doi: 10.1016/S0140-6736(10)60101-8. Retrieved from https://pubmed.ncbi.nlm.nih.gov/20116841/ 
  3. GoodRx.com. (n.d.). Plan B. Next Choice, Levonorgestrel. Retrieved Jan. 12, 2022 from https://www.goodrx.com/ 
  4. Turok, D. (2021). Patient education: Emergency contraception (Beyond the Basics). In C. A. Schreiber & K. Eckler (Eds.). Retrieved Jan. 20, 2022 from https://www.uptodate.com/contents/emergency-contraception-beyond-the-basics 
  5. U.S. Food and Drug Administration (FDA). (2009). Plan B One Step (levonorgestrel) tablet, 1.5 mg, for oral use. Retrieved Jan. 12, 2022 from https://www.accessdata.fda.gov/drugsatfda_docs/label/2009/021998lbl.pdf 
  6. U.S. Food and Drug Administration (FDA). (2015). FDA’s Decision Regarding Plan B: Questions and Answers. Retrieved Jan. 18, 2022 from https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/fdas-decision-regarding-plan-b-questions-and-answers 
  7. U.S. Food and Drug Administration (FDA). (2019). Plan B One Step (levonorgestrel) carton label. Retrieved Jan. 12, 2022 from https://www.accessdata.fda.gov/drugsatfda_docs/label/2019/021998Orig1s006lbl.pdf 
  8. U.S. Food and Drug Administration (FDA). (2021). Ella (ulipristal acetate) tablet, for oral use. Retrieved Jan. 19, 2022 from https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/022474s011lbl.pdf 
  9. World Health Organization (WHO). (2021). Emergency contraception. Retrieved Jan. 12, 2022 from https://www.who.int/news-room/fact-sheets/detail/emergency-contraception

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.