Plan B prevents about 85% of expected pregnancies when taken within 72 hours of unprotected sex, though its effectiveness depends heavily on how quickly you take it and your body weight. It works best in the first 24 hours and becomes less reliable with each passing day.
How Plan B Actually Works
Plan B contains a synthetic hormone called levonorgestrel that primarily stops or delays your ovary from releasing an egg. If ovulation hasn’t happened yet, the drug can pause the process long enough for sperm (which survive up to five days) to die off before an egg becomes available. This is why timing matters so much: if you’ve already ovulated, Plan B has little to offer.
Plan B does not end a pregnancy that has already begun. Once a fertilized egg has implanted in the uterus, the pill has no effect.
Effectiveness by Timing
The sooner you take Plan B after unprotected sex, the better it works. Within the first 24 hours, it prevents the highest proportion of pregnancies. By 72 hours, the pregnancy rate in clinical trials was about 2.6%, meaning roughly 97 out of 100 women who took it did not become pregnant. That number sounds high, but keep in mind that most of those women wouldn’t have become pregnant anyway. The real measure is how many expected pregnancies it prevents, and that figure drops from around 95% in the first 12 hours to roughly 58% by 49 to 72 hours.
You can take Plan B up to five days (120 hours) after sex, but the data beyond three days is not encouraging. In one large trial published in The Lancet, all pregnancies that occurred among women who took emergency contraception between 72 and 120 hours happened in the levonorgestrel (Plan B) group, not the group taking a different pill. After three days, Plan B is essentially unreliable.
Body Weight Changes the Math
Weight is the single biggest factor that can undermine Plan B’s effectiveness, and it’s something the packaging doesn’t make obvious. Research from Oregon Health & Science University found that women with a BMI of 30 or higher experienced Plan B failure four times as often as women with a BMI under 25. The reason is straightforward: blood levels of the drug were about 50% lower in women with a higher BMI after taking a standard dose, meaning the hormone never reached the concentration needed to reliably block ovulation.
Doubling the dose doesn’t fix this. The same research team tested whether taking two Plan B pills would compensate for the lower blood levels in women weighing at least 176 pounds with a BMI over 30. It didn’t work. The double dose still failed to reliably prevent ovulation in that group. For women in this weight range, other emergency contraception options are significantly more effective.
How Plan B Compares to Other Options
Three types of emergency contraception are available in the United States, and they aren’t equally effective.
- Plan B (levonorgestrel): Available over the counter, no prescription needed. Works best within 24 hours, useful up to 72 hours, and unreliable beyond that. Reduced effectiveness at higher body weights.
- Ella (ulipristal acetate): Requires a prescription. In a meta-analysis of clinical trials, the pregnancy rate was 1.4% for Ella compared to 2.2% for Plan B when taken within 72 hours. The real advantage shows up later: Ella remains effective between 72 and 120 hours, while Plan B does not. Ella also works better for women with obesity, according to CDC guidelines.
- Copper IUD: The most effective emergency contraceptive available. It must be inserted by a healthcare provider within five days, but it reduces the risk of pregnancy by more than 99%. It also doubles as ongoing birth control for up to 10 years.
If you’re within the first three days and have a BMI under 25, Plan B and Ella perform similarly. Outside those parameters, Ella or the copper IUD are stronger choices.
Side Effects and What to Expect
Plan B’s side effects are mild and temporary for most people. In clinical trials, about 14% of women experienced nausea and close to 10% reported dizziness. Headache, fatigue, lower abdominal pain, and breast tenderness also occur. If you vomit within two hours of taking the pill, it may not have been absorbed, and you’d need another dose.
Your next period will likely be different. It may arrive up to a week early or a week late, and the flow can be heavier or lighter than usual. Some women get an early bleed and then another period at the expected time, which can be confusing but is normal. If your period is more than a week late, a pregnancy test is a reasonable next step.
There are no known long-term health effects from taking Plan B, and using it more than once in the same cycle is not dangerous. However, it’s not designed as a regular contraceptive method. Repeated use doesn’t cause harm, but it’s less effective and more expensive than ongoing birth control.
Why the “89% Effective” Number Is Misleading
You’ll often see Plan B described as “up to 89% effective,” but that number needs context. It doesn’t mean 11 out of 100 women who take it will get pregnant. It means that among women who would have become pregnant without any intervention, Plan B prevents about 89% of those pregnancies when taken within 72 hours. Your actual chance of pregnancy from a single act of unprotected sex is relatively low to begin with (around 5 to 8% on average, higher near ovulation), so Plan B is reducing an already small risk.
The flip side is that Plan B is far from a guarantee. If you happened to have sex right before ovulation, your baseline pregnancy risk is at its highest, and Plan B may have already missed its window to block egg release. In that scenario, the pill offers much less protection than the overall statistics suggest.