The morning after pill prevents roughly 85% to 95% of expected pregnancies, but that range depends heavily on how quickly you take it. At 24 hours, the most common type is about 94% effective. By 72 hours, that number drops to around 58%. Timing is the single biggest factor in whether it works.
How Timing Changes Effectiveness
There are two types of morning after pill available, and they have different time windows. The most common one, sold over the counter as Plan B and its generics, contains levonorgestrel. It’s labeled for use up to 72 hours (three days) after unprotected sex but works best when taken as early as possible. Studies show it’s around 94% effective within the first 24 hours. By 72 hours, effectiveness falls to about 58%.
The second type, sold under the brand name Ella, contains ulipristal acetate and requires a prescription. It works for up to 120 hours (five days) after unprotected sex. Within the first three days, Ella and Plan B perform similarly. The key difference is that Ella maintains its effectiveness between days three and five, while Plan B drops off sharply. In clinical studies, both types showed pregnancy rates between 1.2% and 2.1% when used within their recommended windows, which means the vast majority of women who use them correctly do not become pregnant.
Those percentages describe how many women got pregnant compared to how many would have been expected to get pregnant without any emergency contraception. The absolute pregnancy rate per use is low, but the protection gets weaker with every hour you wait.
How the Morning After Pill Works
Both types of morning after pill work primarily by delaying or preventing ovulation. If your body hasn’t yet released an egg, the pill can pause that process long enough for sperm to die off (sperm survive about five days in the reproductive tract). Once the window closes, fertilization can’t happen.
This mechanism explains why timing matters so much. If you’ve already ovulated, the pill has little to no effect. It does not end an existing pregnancy. The closer you are to ovulation when you take it, the less likely it is to work, because the hormonal surge that triggers egg release may already be too far along to stop.
Body Weight Can Reduce Effectiveness
Research has found that emergency contraception becomes less reliable at higher body weights. One significant finding showed that pills seemed less likely to work for women with a BMI above 26, which corresponds to roughly 155 pounds for someone 5’4″. Plan B (levonorgestrel) is the most affected by this. Ella appears to be impacted by weight too, but not to the same extent, making it the better pill option for women with a higher BMI.
If you’re concerned about weight affecting effectiveness, there’s a third option worth knowing about. A copper IUD inserted within 120 hours of unprotected sex is more than 99% effective at preventing pregnancy regardless of body weight. It’s the most effective form of emergency contraception available and doubles as long-term birth control for up to 10 years. The tradeoff is that it requires a clinical visit for insertion.
Medications That Interfere
Certain medications speed up how quickly your liver breaks down levonorgestrel, which can lower the amount in your bloodstream enough to reduce effectiveness. The main culprits are some medications used to treat epilepsy, tuberculosis, HIV, and fungal infections. The herbal supplement St. John’s Wort has the same effect. Even if you stopped taking one of these medications recently, the interaction can persist for up to four weeks.
If you’ve used any of these in the past month, the standard recommendation is to double the levonorgestrel dose (two packs of Plan B instead of one). Alternatively, a copper IUD avoids the interaction entirely since it doesn’t rely on hormones to work.
What Can Go Wrong After Taking It
If you vomit within two hours of taking a morning after pill, your body may not have absorbed enough of the medication. You’ll likely need to take another dose. Common side effects like nausea, headache, fatigue, and irregular bleeding are temporary and not a sign the pill didn’t work.
Having unprotected sex again after taking emergency contraception is one of the strongest predictors of the pill failing. A single dose protects against the one episode that already happened. It does not provide ongoing coverage for the rest of your cycle. If you have sex again before your next period, you need a regular contraceptive method.
Using emergency contraception more than once in a cycle is considered safe, but the data on how well it works in that scenario is limited. It’s not designed as a routine method of birth control, and regular contraception is significantly more reliable over time.
How to Know If It Worked
Your next period is the main indicator. It may arrive a few days early or late, which is normal. If your period is more than a week late, take a pregnancy test. Lower abdominal pain or persistent irregular bleeding after using emergency contraception also warrants a check, since these can occasionally signal a problem that needs evaluation.
Emergency contraception reduces the overall risk of pregnancy, including ectopic pregnancy (where an embryo implants outside the uterus). It does not increase ectopic risk, despite a common misconception.