The morning after pill is a form of emergency contraception you take after unprotected sex to prevent pregnancy. It works by delaying or preventing ovulation, the release of an egg from the ovaries. It does not end an existing pregnancy. Two types are available in the United States: one sold over the counter at most pharmacies, and another that requires a prescription.
How It Works
Both types of morning after pill share the same core mechanism: they stop or delay your ovary from releasing an egg. If there’s no egg available, sperm can’t fertilize anything, and pregnancy doesn’t happen. This is the same basic principle behind daily hormonal birth control, just delivered in a single, higher dose designed for one-time use after the fact.
The morning after pill does not induce an abortion. If a fertilized egg has already implanted in the uterus, the pill will not disrupt that pregnancy.
The Two Types Available
The over-the-counter option contains levonorgestrel, a synthetic form of progesterone. You’ll find it under brand names like Plan B One-Step, Take Action, My Way, Aftera, and several other generics. No prescription is needed, and there are no age restrictions. You can buy it at most drugstores or online, typically for $11 to $50 depending on the brand.
The prescription option contains ulipristal acetate, sold under the brand name Ella. It requires a prescription for all ages. Ella works somewhat differently: in addition to delaying ovulation, its anti-progesterone properties give it a wider effective window, which matters if several days have passed since unprotected sex.
How Quickly You Need to Take It
Timing is the single biggest factor in how well the morning after pill works. Levonorgestrel pills (Plan B and generics) are 81 to 90 percent effective at preventing pregnancy, with that number depending almost entirely on how soon you take them. They work best within 72 hours (three days) of unprotected sex, and you can still use them up to five days afterward, though effectiveness drops noticeably past the three-day mark. Taking it within the first 24 hours gives you the best odds.
Ella can be taken up to five days (120 hours) after unprotected sex and maintains its effectiveness more consistently across that full window. If you’re past the 72-hour mark, Ella is the stronger option, though you’ll need to get a prescription first.
Body Weight and Effectiveness
Research from Oregon Health & Science University found that emergency contraception becomes less effective for people with a BMI above 26. Levonorgestrel is the most likely of the two options to fail at higher body weights. Ulipristal acetate (Ella) may also be affected by weight, but not to the same degree, making it the better choice for people with a higher BMI.
A copper IUD, inserted by a healthcare provider within five days of unprotected sex, is another emergency contraception option that is not affected by body weight. It’s the most effective form of emergency contraception available and doubles as long-term birth control for up to 10 years afterward.
Common Side Effects
The morning after pill can cause nausea, headache, fatigue, dizziness, and breast tenderness in the hours and days after you take it. Some people experience light spotting or bleeding. These effects are temporary and typically resolve within a day or two.
Your next period may arrive earlier or later than expected, and the flow may be lighter or heavier than usual. This is normal. If your period is more than a week late, take a pregnancy test. The NHS recommends testing at least 21 days after unprotected sex for a reliable result, or from the first day of a missed period if you know when it’s due.
Restarting Regular Birth Control Afterward
If you took a levonorgestrel pill (Plan B or a generic), you can start or restart your regular hormonal birth control right away. Use a backup method like condoms for the next seven days while your regular contraception takes effect.
If you took Ella, the timing is different and important. Because Ella has anti-progesterone properties, starting hormonal birth control too soon can interfere with how well both medications work. The CDC recommends waiting at least five days after taking Ella before starting or resuming hormonal contraception. During that waiting period, and for seven days after restarting your regular method, use condoms or abstain from sex.
What the Morning After Pill Is Not
The morning after pill is not a replacement for regular contraception. It delivers a higher hormone dose than daily birth control and is designed for occasional, emergency use. It’s also less effective than most ongoing methods. Used perfectly, daily birth control pills, IUDs, and implants all prevent pregnancy at significantly higher rates than emergency contraception taken after the fact.
Taking it more than once won’t cause long-term health problems, but relying on it repeatedly is a signal that a regular contraceptive method would serve you better and work more reliably.