Emergency contraception is a way to prevent pregnancy after unprotected sex or a contraceptive failure, such as a broken condom or a missed birth control pill. It is not the same as an abortion. There are four options available in the United States: three types of emergency contraceptive pills and the copper intrauterine device (IUD). All of them work best the sooner you use them, but depending on the method, you may have up to five days.
How Emergency Contraception Works
The primary way emergency contraceptive pills prevent pregnancy is by delaying or blocking ovulation, the release of an egg from the ovary. If no egg is released, sperm have nothing to fertilize, and pregnancy cannot begin. This is an important distinction: emergency contraception only works if you are not already pregnant. It does not end an existing pregnancy, which is what sets it apart from medication abortion.
The two main pill options delay ovulation in slightly different ways and at different points in your cycle. Levonorgestrel, the active ingredient in Plan B and its generics, is effective at blocking ovulation when taken early in the fertile window. But once the hormonal surge that triggers ovulation has already started, levonorgestrel prevents ovulation in only about 12% of cycles, essentially no better than a placebo. Ulipristal acetate (sold as ella) works differently. It can block ovulation even after that hormonal surge has begun, succeeding in about 79% of cycles at that stage compared to 14% for levonorgestrel. This is why ulipristal acetate remains effective closer to the moment of ovulation, when the chance of conception is highest.
The copper IUD prevents pregnancy through a different mechanism. Copper ions create an environment in the uterus that is toxic to sperm, preventing fertilization. Because it doesn’t rely on delaying ovulation, it is the most effective emergency contraceptive option available.
Your Options at a Glance
Levonorgestrel pills (Plan B and generics) are available over the counter to anyone of any age, with no ID required. They work best when taken as soon as possible and are generally recommended within 72 hours (3 days) of unprotected sex, though some effectiveness remains up to 5 days. Effectiveness drops noticeably with each passing day.
Ulipristal acetate (ella) requires a prescription. It maintains its effectiveness better than levonorgestrel over the full 5-day window, which is one of its key advantages. The American College of Obstetricians and Gynecologists notes that ulipristal acetate is more effective than levonorgestrel at all time points up to 5 days and across all body weights.
The copper IUD must be inserted by a healthcare provider within 5 days of unprotected sex. It fails to prevent pregnancy in fewer than 0.1% of cases, making it roughly ten times more effective than oral methods. A major bonus: once placed, it provides up to 10 to 12 years of ongoing contraception.
The hormonal (levonorgestrel) IUD is a newer option for emergency contraception. A 2021 trial published in the New England Journal of Medicine found it was noninferior to the copper IUD, with a pregnancy rate of 0.3% compared to 0% for copper. Like the copper IUD, it doubles as long-term birth control after insertion.
Body Weight and Effectiveness
Both levonorgestrel and ulipristal acetate may become less effective for people who are overweight (BMI 25 to 29.9) or obese (BMI 30 or higher). Levonorgestrel appears to lose efficacy at a lower weight threshold than ulipristal acetate, which is another reason ulipristal acetate is often the preferred pill option. For people at higher body weights, the copper IUD is the most reliable choice because its effectiveness does not depend on body weight.
Side Effects
Emergency contraceptive pills are generally well tolerated. The most common side effects of levonorgestrel-based pills are nausea (about 23% of users), abdominal pain (18%), fatigue or headache (17%), and vomiting (6%). Breast tenderness and dizziness are less common. These side effects are typically short-lived, resolving within a day or two.
Your next period may shift slightly. About 58% of people get their period within a few days of the expected date. Most will have a period within 21 days of treatment. Some light spotting in the days after taking the pill is normal and not a sign of a problem.
Can You Use It More Than Once?
Using emergency contraceptive pills more than once in the same menstrual cycle does not appear to pose serious health risks, though the evidence is limited. Studies of repeated levonorgestrel use found few serious adverse events, and studies of repeated ulipristal acetate use (up to 4 to 6 doses per month) showed no serious adverse events, no abnormal lab results, and normal uterine lining on biopsy. That said, emergency contraception is less effective than regular contraceptive methods used consistently. If you find yourself reaching for it frequently, a daily pill, an IUD, an implant, or another ongoing method will give you more reliable protection.
Breastfeeding and Emergency Contraception
Levonorgestrel pills are considered safe during breastfeeding. Ulipristal acetate passes into breast milk in small amounts, but current FDA labeling no longer requires pumping and discarding milk or pausing breastfeeding after a dose. Older sources may still recommend a 24-hour pause, but this guidance has been updated. The copper IUD is also safe for breastfeeding individuals.
How to Get It
Levonorgestrel pills (Plan B, Take Action, and other generics) are sold at pharmacies, many grocery stores, and online without a prescription or age restriction. They typically cost between $10 and $50 depending on the brand. Ulipristal acetate requires a prescription from a healthcare provider, though some telehealth services can prescribe it quickly. For either IUD option, you’ll need an appointment with a provider who can insert it within the 5-day window, which may require calling ahead to confirm availability.
Timing matters more than anything else with emergency contraception. If you think you need it, taking action the same day rather than waiting will always give you the best chance of preventing pregnancy.