The morning after pill contains a single hormone as its active ingredient. The most common version, sold as Plan B One-Step and its generics, contains 1.5 mg of levonorgestrel, a synthetic form of progesterone that has been used in birth control pills for decades. A prescription alternative called ella contains a different compound, ulipristal acetate, at a dose of 30 mg. Both are single-tablet pills, and each works by interfering with ovulation to prevent pregnancy.
Levonorgestrel: The Over-the-Counter Option
Levonorgestrel is classified as a synthetic progestogen, meaning it mimics the natural hormone progesterone your body already produces during your menstrual cycle. At the 1.5 mg dose found in Plan B and its generics, it’s roughly 10 times the amount of levonorgestrel in a single daily birth control pill. That concentrated dose is what allows it to work as a one-time emergency option rather than something you take every day.
This pill is available without a prescription at most pharmacies. It works best when taken as soon as possible after unprotected sex. Taken within the first 24 hours, it’s about 94% effective at preventing pregnancy. By the 72-hour mark (three days), effectiveness drops to around 58%. The window technically extends to 72 hours, but the sooner you take it, the better it works.
Ulipristal Acetate: The Prescription Alternative
The other type of morning after pill, sold under the brand name ella, contains 30 mg of ulipristal acetate. Rather than simply mimicking progesterone like levonorgestrel does, ulipristal acetate is a selective progesterone receptor modulator. It both activates and blocks progesterone receptors depending on the tissue, which gives it a stronger ability to delay ovulation even when your body is closer to releasing an egg.
This matters for timing. Ella remains effective for up to five days (120 hours) after unprotected sex, giving it a longer useful window than levonorgestrel. It also holds up better for women with a higher body weight. The tradeoff is that ella requires a prescription.
How They Actually Prevent Pregnancy
Both pills work primarily by delaying or preventing ovulation. If your ovary hasn’t released an egg yet, the hormone signal from the pill can pause that process long enough for sperm to die off (sperm survive about five days in the reproductive tract). No egg released means no fertilization.
A common concern is whether the morning after pill affects a fertilized egg’s ability to implant in the uterus. Research on levonorgestrel is clear on this point. A review in the journal Contraception found that levonorgestrel taken after ovulation does not affect implantation and results in conception rates similar to placebo. Studies on human endometrial tissue showed no change in the markers that indicate whether the uterine lining is receptive to an embryo. In other words, if ovulation has already happened, levonorgestrel doesn’t appear to interfere with what comes next.
This is why timing matters so much. The pill can only do its job if it reaches your system before the egg is released. Once ovulation has occurred, its ability to prevent pregnancy drops significantly.
Body Weight and Effectiveness
Both types of morning after pill may be less effective for women who are overweight (BMI of 25 to 29.9) or obese (BMI of 30 or higher). Levonorgestrel appears to lose efficacy at a lower weight threshold than ulipristal acetate, making ella the better pill option for women at higher body weights.
For women of any weight who want the most reliable emergency contraception, a copper IUD inserted within five days of unprotected sex is the most effective option available. In a study of 318 participants who received a copper IUD for emergency contraception, zero pregnancies occurred, a 100% prevention rate. A hormonal IUD containing levonorgestrel also showed strong results, preventing 93% to 96% of expected pregnancies.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down levonorgestrel, which can reduce the amount circulating in your body enough to make the pill less effective. These include medications used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampicin, some HIV medications including efavirenz, and the antifungal griseofulvin. St. John’s wort, a common herbal supplement, also reduces levonorgestrel levels.
This interaction isn’t limited to drugs you’re currently taking. Elevated enzyme levels can persist for up to four weeks after stopping one of these medications. If you’ve used any of them in the past month, the standard levonorgestrel dose may not be enough. In that situation, U.K. prescribing guidance recommends doubling the dose from 1.5 mg to 3 mg (two packs) to compensate, or using a copper IUD instead.
Common Side Effects
Because the morning after pill delivers a concentrated dose of hormone, short-term side effects are common but generally mild. The most frequently reported include nausea, headache, fatigue, dizziness, and breast tenderness. Some women experience heavier or lighter bleeding than usual during their next period, or their period may arrive a few days early or late. Cramping and spotting in the days after taking the pill are also normal.
These effects typically resolve within a day or two. The pill does not cause long-term changes to your fertility or your menstrual cycle beyond the current month. It also does not end an existing pregnancy. If you’re already pregnant when you take it, the pill simply won’t work.