Morning after pill: how does emergency contraception work?

last updated: Oct 19, 2023

4 min read

Maybe the condom broke. Maybe you missed a few birth control pills. No matter the reason, the morning-after pill is an effective option for helping to prevent pregnancy after unprotected sex. Read on to learn more about the morning-after pill, how it works, how to use it, and how effective it is. 

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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, EC is used after unprotected sex to help reduce the chance of becoming pregnant. 

EC isn’t typically used regularly like other birth control methods. Instead, it’s usually meant to be used if your regular birth control method failed (like if you forgot to take a few birth control pills or if the condom broke). 

Keep in mind that even though it’s called the morning-after pill, you don’t need to take it the morning after unprotected sex. In the case of levonorgestrel (brands include Plan B, MyWay, and others), you can take it up to 72 hours after unprotected sex, but the sooner you take it, the better. (If you don’t have any other options, some healthcare providers recommend trying it up to five days after unprotected sex, though it’s less likely to work). Ulipristal (brand name Ella), which requires a prescription from a healthcare provider, can be effective for up to five days after unprotected sex. 

Do I need a prescription for the morning-after pill?

While levonorgestrel (Plan B) is available in all 50 states without a prescription, ulipristal (Ella) requires a prescription from a healthcare provider. You do not need to wait until you have an emergency to get emergency contraception; you can get it ahead of time and keep it on hand just in case, just like Advil and tampons.

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. Since Ella is a prescription-only drug, many insurance plans will cover it. And even though you don’t need a prescription for Plan B, some medical assistance and insurance plans will cover it if you have 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 helps prevent pregnancy by temporarily stopping or delaying ovulation, which is the release of an egg from the ovaries.

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 pregnancy. That’s because EC may help prevent a fertilized egg from implanting into the uterus.

Emergency contraception medications won’t have any effect if you’re already pregnant. They are not abortion pills; so they can’t stop a pregnancy or harm an unborn baby.

Keep in mind that one dose of emergency contraception works for one encounter of unprotected sex and won’t work for an extended period of time. It’s important to note that Ella, the prescription morning-after pill, can actually interfere with regular hormonal birth control so you’ll need to use a back up barrier method (like a condom) until your next period. Also, the morning-after pill does not protect against sexually transmitted infections (STIs).

How effective is the morning-after pill?

The data show that about seven out of eight people who use the morning-after pill after unprotected sex avoid pregnancy. 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, ideally within three 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. 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:

OC Morning after pill: how does emergency contraception work? image 954703c9-ba3a-49c6-be20-c154978d209f

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

Also, if you’re already using hormonal birth control (like the pill, patch, or vaginal ring), taking ulipristal 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 . 

Levonorgestrel (Plan B) may be slightly less effective in women with obesity, defined as having a body mass index (BMI) greater than 30. 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.

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:

  • 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.


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.

How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

October 19, 2023

Written by

Patricia Weiser, PharmD

Fact checked by

Yael Cooperman, MD

About the medical reviewer

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.