The morning-after pill can help prevent pregnancy in people who:

• Didn't use their regular type of birth control, such as condoms, during sex.
• Missed doses of daily birth control pills.
• Were sexually assaulted.
• Used birth control that didn't work. For example, condoms can break or slip off by accident during sex.

Morning-after pills work mainly by delaying or preventing the release of an egg from the ovaries, called ovulation. They do not end a pregnancy that has already started. Different medicines are used to end an early pregnancy in a treatment called a medical abortion. Medicines used in a medical abortion can include mifepristone (Mifeprex, Korlym) and misoprostol (Cytotec).

Emergency birth control is an effective way to prevent pregnancy after sex without protection. But it doesn't work as well as other types of birth control. And emergency contraception isn't meant for routine use. Also, the morning-after pill might not work even if you use it correctly. And it doesn't protect you from sexually transmitted infections.

The morning-after pill isn't right for everyone. Don't take a morning-after pill if:

• You're allergic to any ingredient in it.
• You take certain medicines that can affect how well the morning-after pill works, such as barbiturates and St. John's wort.

If you're overweight or obese, the morning-after pill might not work as well as it would for people who aren't overweight.

Also, make sure you're not pregnant before using ulipristal. The effects of ulipristal on a developing baby aren't known. If you're breastfeeding, do not take ulipristal.

Side effects of the morning-after pill often last only a few days. They can include:

• Upset stomach or vomiting.
• Dizziness.
• Fatigue.
• Headache.
• Tender breasts.
• Light bleeding between periods or heavier menstrual bleeding.
• Pain or cramps in the stomach area.

For the morning-after pill to work best, take it as soon as possible after sex without protection. You need to use it within five days, or 120 hours, for it to work. You can take emergency birth control pills anytime during your menstrual cycle.

To use the morning-after pill:

• Follow the morning-after pill's instructions. If you use Plan B One-Step, take one Plan B One-Step pill as soon as possible after unprotected sex. It works best if you take it within three days, or 72 hours. But it can still be effective if you take it within five days, or 120 hours. If you use ella, take one ella pill as soon as possible within five days.
• If you vomit within three hours after taking the morning-after pill, ask your healthcare professional if you should take another dose.
• Don't have sex until you start another type of birth control. The morning-after pill doesn't offer lasting protection from pregnancy. If you have sex without protection in the days and weeks after taking the morning-after pill, you're at risk of becoming pregnant. Be sure to start using or resume use of birth control.

Using the morning-after pill may delay your period by up to one week. If you don't get your period within three weeks of taking the morning-after pill, take a pregnancy test.

Most often, you don't need to contact your healthcare professional after using the morning-after pill. But you should call your healthcare professional if you have any of the following symptoms:

• Heavy bleeding with pain in the stomach area.
• Ongoing spotting or irregular bleeding.

These can be symptoms of a miscarriage. These also can be symptoms of a pregnancy that forms outside the uterus, called an ectopic pregnancy. Without treatment, an ectopic pregnancy can be life-threatening for someone who's pregnant.