Emergency contraceptive pills are medications taken after unprotected sex to prevent pregnancy. They work primarily by delaying or blocking ovulation, and they come in two main types: one available over the counter and one requiring a prescription. Neither type ends an existing pregnancy, and both are most effective the sooner you take them.
The Two Types of Pills
The first and most widely known type contains levonorgestrel, a synthetic hormone. It’s sold under brand names like Plan B One-Step and several generics. You can buy it over the counter at pharmacies without a prescription and without age restrictions. The standard dose is a single 1.5 mg tablet.
The second type contains ulipristal acetate, sold as ella. This one requires a prescription. It works as a single 30 mg tablet and is more effective than levonorgestrel across the entire window of use. The tradeoff is that you need to contact a healthcare provider or use a telehealth service to get it.
How They Prevent Pregnancy
Both pills work mainly by interfering with ovulation. If your body hasn’t yet released an egg, the medication delays or stops that release, which means sperm waiting in the reproductive tract never encounter an egg to fertilize. Research has shown that levonorgestrel is essentially 100% effective at preventing pregnancy when taken before ovulation, but has no measurable effect if taken on the day of ovulation or after. This is why timing matters so much.
There’s also evidence that these pills may alter conditions in the fallopian tubes, slowing the transport of sperm or eggs in ways that make fertilization less likely. Ulipristal acetate can block ovulation at a later stage in the hormonal cycle than levonorgestrel can, which is part of why it remains effective for a longer window.
How Quickly You Need to Take Them
Levonorgestrel (Plan B) can be taken up to 72 hours (three days) after unprotected sex, but effectiveness drops with each passing hour. Ulipristal acetate (ella) extends that window to 120 hours (five days) and is more effective than Plan B at every time point within that range. For either pill, sooner is always better. Taking levonorgestrel within the first 24 hours gives you the best chance of preventing pregnancy.
How Well They Work
Neither pill is as reliable as ongoing birth control. Levonorgestrel-based pills have a pregnancy rate of roughly 2 to 3%, while ulipristal acetate brings that down to about 1.4%. For comparison, a copper IUD inserted as emergency contraception has a failure rate of just 0.09%, making it by far the most effective emergency option, with over 99% of users avoiding pregnancy. The copper IUD can also be placed up to five days after unprotected sex and then serves as long-term contraception.
Weight and Effectiveness
Body weight significantly affects how well these pills work, and this is an area many people aren’t aware of. Levonorgestrel appears to hit a ceiling of effectiveness at around 70 kg (about 154 pounds) and may have essentially no efficacy for people weighing 80 kg (176 pounds) or more. People with a BMI of 30 or higher who use levonorgestrel have roughly four times the risk of pregnancy compared to those with a BMI under 25.
Ulipristal acetate holds up somewhat better at higher weights, though its effectiveness also decreases. If you weigh over 155 pounds, ulipristal acetate is the stronger choice of the two pills. If you weigh over 176 pounds, a copper IUD is the most reliable emergency contraception option available.
Common Side Effects
The most common side effect of levonorgestrel pills is nausea, which affects about 23% of users. Around 18% experience abdominal pain, 17% report fatigue or headache, and about 6% vomit. Breast tenderness and dizziness occur less frequently. These side effects are temporary, typically 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 their period within 21 days of taking the pill. Some light spotting in the days after treatment is normal and not a sign that anything is wrong. If your period is more than a week late, it’s worth taking a pregnancy test.
Safety Considerations
Emergency contraceptive pills do not harm an existing pregnancy. If you’re already pregnant and don’t know it, taking one of these pills will not cause a miscarriage or harm the embryo. They only work by preventing pregnancy from occurring in the first place.
A systematic review of the evidence found that emergency contraceptive pills do not increase the risk of ectopic pregnancy (a pregnancy that implants outside the uterus). The ectopic pregnancy rate among people who used these pills was 0.6 to 1%, which falls within or below the 0.8 to 2% range seen in the general population. Because the pills reduce your overall chance of becoming pregnant, they actually lower your absolute risk of ectopic pregnancy after any given act of unprotected sex.
For breastfeeding parents, levonorgestrel is considered compatible with nursing. Ulipristal acetate passes into breast milk in low amounts. Older guidelines recommended pumping and discarding milk for 24 hours after taking it, but current FDA labeling no longer requires that, and breastfeeding does not need to be discontinued.
The Copper IUD as an Alternative
If you’re within five days of unprotected sex and can get an appointment quickly, a copper IUD is the most effective form of emergency contraception available. It prevents over 99% of pregnancies, works regardless of body weight, and then provides up to 10 years of ongoing contraception. The downside is that it requires a clinical visit for insertion, which isn’t always possible on short notice. For people who want both emergency and long-term protection, it’s worth considering.