You can take Plan B up to 72 hours (3 days) after unprotected sex, but it works significantly better the sooner you take it. Within the first 24 hours, it’s about 94% effective. By the 72-hour mark, that drops to roughly 58%. Every hour you wait reduces your protection, so treating it as a “sooner is better” situation rather than a “3-day window” is the smarter approach.
How Effectiveness Changes Hour by Hour
The FDA recommends taking Plan B as soon as possible within 72 hours. That 72-hour cutoff is the official limit on the label, but it’s not a cliff where the pill suddenly stops working. It’s more of a steep slide. At 94% effectiveness in the first 24 hours, Plan B prevents the vast majority of pregnancies. Between 24 and 48 hours, effectiveness is lower but still meaningful. By 48 to 72 hours, you’re down to about 58% effectiveness, which means it’s still reducing your risk by more than half compared to doing nothing, but it’s far less reliable than it would have been a day or two earlier.
Some studies suggest Plan B may offer a small degree of protection even past the 72-hour window, but the data isn’t strong enough to count on it. If you’re already past 72 hours, a different option will serve you better.
Why Timing Matters So Much
Plan B works by temporarily stopping your ovary from releasing an egg. Think of it as hitting a pause button on ovulation. If ovulation hasn’t happened yet, the pill can delay it long enough for sperm (which survive up to five days) to die off before they ever encounter an egg. No egg, no fertilization, no pregnancy.
The reason timing is critical: once ovulation has already occurred, Plan B has little to no effect. It does not prevent a fertilized egg from implanting, and it will not end an existing pregnancy. So the clock you’re racing against isn’t just the 72-hour label. It’s your own ovulation cycle. If you happened to have sex right before or during ovulation, the pill is less likely to help regardless of when you take it. But since most people can’t pinpoint their exact ovulation day, taking it as fast as possible gives you the best odds of catching it in time.
Options If You’re Past 72 Hours
If more than three days have passed, you still have effective choices. The most important one to know about is a prescription emergency contraceptive called ella, which contains a different active ingredient and works for up to 120 hours (5 full days) after unprotected sex. In clinical trials, ella reduced the expected pregnancy rate from 5.6% to 1.9% when taken within 72 hours. Among women who took it between 72 and 120 hours, no pregnancies were observed. Unlike Plan B, ella showed no significant drop in effectiveness across the entire five-day window, making it a far better choice when you’re on day four or five.
Ella requires a prescription, but many telehealth services can provide one quickly, sometimes within hours. Some pharmacies also carry it behind the counter.
The copper IUD is the most effective form of emergency contraception available. Inserted by a healthcare provider within five days of unprotected sex, it’s highly effective at preventing pregnancy and can then stay in place as long-term birth control for up to 10 years. It’s worth considering if you also want ongoing contraception.
Body Weight Can Affect How Well It Works
This is something many people don’t hear about, but research shows that Plan B becomes less effective at higher body weights. Studies indicate that its effectiveness starts to decline noticeably at around 70 kg (about 155 pounds) and may provide essentially no protection at 80 kg (about 176 pounds) or above. The reason is pharmacological: at higher weights, the body reaches lower peak concentrations of the drug, roughly 50% lower in people with a BMI of 30 or above compared to those under 25.
If your weight is in this range, ella is a better choice for oral emergency contraception. While obesity can also reduce ella’s effectiveness somewhat, the impact is less dramatic than with Plan B. The copper IUD remains equally effective regardless of body weight, making it the most reliable option for everyone.
What to Expect Afterward
After taking Plan B, your next period may arrive up to a week earlier or later than expected. This is normal and happens because the hormone dose temporarily disrupts your cycle’s timing. Some people experience nausea, headache, fatigue, or light spotting in the days after taking it. These effects are typically mild and short-lived.
If your period is more than a week late after taking Plan B, take a pregnancy test. A late period doesn’t necessarily mean the pill failed, but it’s worth confirming. Plan B is a single-use backup, not a sign that something went wrong with your body. Your cycle should return to its normal pattern within one to two months.