Plan B is most effective within the first 24 hours after unprotected sex, when it reduces the risk of pregnancy by about 94%. Its effectiveness drops significantly over the next two days, falling to roughly 58% by the 72-hour mark. After three days, it is not considered reliable.
The 72-Hour Window, Hour by Hour
Plan B has a hard deadline: 72 hours (three days) after unprotected sex. But treating that as a flat window is misleading, because the difference between taking it at hour 6 and hour 60 is enormous. At 24 hours, it prevents about 94% of expected pregnancies. By 48 hours, effectiveness has already dropped noticeably. At 72 hours, it prevents only about 58% of expected pregnancies. Every hour you wait reduces your protection, so taking it as soon as possible is the single most important thing you can do.
How Plan B Actually Prevents Pregnancy
Plan B contains a large dose of a synthetic hormone that works by delaying or blocking ovulation. If your body hasn’t yet released an egg, the hormone postpones that release by at least five days, which is long enough for sperm (which survive about five days) to die off before an egg becomes available. No egg, no fertilization.
This is also why timing matters so much. If you’ve already ovulated, Plan B has little to no effect. A 2022 review published in the journal Contraception confirmed that when women took the pill at or after the hormonal surge that triggers ovulation, their conception rates were similar to women who used no emergency contraception at all. Plan B does not prevent a fertilized egg from implanting, and it does not end an existing pregnancy.
How Long It Stays in Your System
The hormone in Plan B reaches its peak level in your blood about 1.7 hours after swallowing the pill. From there, it has an elimination half-life of roughly 27.5 hours, meaning your body clears half of it in just over a day. After a few days, levels are low enough to be functionally irrelevant. This is important because Plan B does not provide ongoing contraceptive protection. It addresses a single act of unprotected sex, not anything that happens afterward.
If you have unprotected sex again after taking Plan B, even later the same day, you are not protected from that second exposure. You would need another dose or a different form of contraception.
Weight Can Reduce Effectiveness
Body weight plays a significant role in how well Plan B works. Research from large randomized trials found that people with a BMI over 30 were three times as likely to become pregnant after taking Plan B compared to those with a BMI under 25. About 5.8% of participants in the higher BMI group became pregnant, versus 1.3% in the lower BMI group. Researchers identified 155 pounds as the approximate threshold where effectiveness begins to drop sharply.
If your weight is above that range, a prescription alternative called ella (ulipristal acetate) maintains better effectiveness up to about 195 pounds. It also works within a longer window of five days rather than three, and outperforms Plan B at every time point within that window. The most effective emergency option regardless of weight is a copper IUD, which can be placed up to five days after unprotected sex.
Taking Plan B More Than Once
Taking Plan B multiple times in the same menstrual cycle is safe and does not reduce its effectiveness for future use. Each dose works independently. That said, it can temporarily disrupt your cycle, causing your period to arrive earlier or later than expected, and repeated use may make those disruptions more noticeable. It is not harmful as a repeated measure, but it is less effective and more expensive than using a regular contraceptive method.
What to Expect After Taking It
Common side effects include nausea, fatigue, headache, and breast tenderness. These typically resolve within a day or two. Your next period may come up to a week early or a week late. If it’s more than a week late, a pregnancy test is a good idea, since Plan B is not 100% effective even under ideal circumstances.
Irregular bleeding or spotting in the days after taking the pill is also normal. This is not your period and does not indicate whether the pill worked. The only reliable way to confirm you’re not pregnant is a test taken at least three weeks after the unprotected sex, or after a missed period.