Plan B can be taken up to 72 hours (3 days) after unprotected sex, but it works best the sooner you take it. If you’re past that 72-hour mark, you still have options that work up to 120 hours (5 days), including a prescription pill called ella and the copper IUD.
The 72-Hour Window for Plan B
Plan B contains a single hormone that primarily works by delaying ovulation. If your body hasn’t released an egg yet, the pill can pause that process long enough for sperm to die off before they have a chance to fertilize anything. This is why timing matters so much: once ovulation has already happened, Plan B’s ability to prevent pregnancy drops sharply. Studies show that women who took levonorgestrel (the active ingredient in Plan B) on the day of ovulation or after became pregnant at the same rate as women who used no contraception at all.
The effectiveness within that 72-hour window isn’t flat. Taking it within the first 24 hours gives you the best odds. Each day you wait, the chance of pregnancy increases. By hour 72, Plan B still reduces your risk, but noticeably less than if you’d taken it right away. The bottom line: don’t wait if you have the pill available now.
What Works Between 72 and 120 Hours
If you’re reading this and it’s been more than three days, Plan B is no longer recommended. But two other options cover you up to five full days.
Ella (ulipristal acetate) is a prescription emergency contraceptive pill that works up to 120 hours after unprotected sex. It’s more effective than Plan B even within the first 72 hours, and it maintains its effectiveness in the later window where Plan B does not. In a clinical trial comparing the two, no pregnancies occurred among women who took ella between 72 and 120 hours after intercourse. Ella requires a prescription, but many telehealth services can provide one quickly.
The copper IUD can be inserted up to five days after unprotected sex and reduces the risk of pregnancy by over 99 percent. It’s the single most effective form of emergency contraception available. According to 2024 CDC guidelines, if the day of ovulation can be estimated, the copper IUD can even be placed more than five days after sex, as long as it’s inserted within five days of ovulation. The added benefit: once it’s in, it provides ongoing contraception for up to 10 years.
Why Body Weight Can Matter
Plan B may be less effective at higher body weights. U.K. clinical guidelines suggest that women weighing over 70 kg (about 154 pounds) or with a BMI over 26 may want to consider a double dose of the standard 1.5 mg pill. The American Society for Emergency Contraception recommends a double dose for women with a BMI over 30. If your weight falls into these ranges, ella or the copper IUD may be more reliable choices, since their effectiveness isn’t reduced by body weight the same way.
Where You Are in Your Cycle Changes Everything
Plan B’s effectiveness depends not just on how many hours have passed, but on where you are in your menstrual cycle. The pill works by blocking the hormonal surge that triggers ovulation. If that surge hasn’t started yet, Plan B can successfully delay egg release. But if the surge is already underway or ovulation has already occurred, the pill has little to no effect.
This means two women could take Plan B at the exact same number of hours after sex and have very different outcomes, purely based on their cycle timing. If you’re mid-cycle (roughly days 10 through 16 in a typical 28-day cycle), that’s when ovulation is most likely and when timing becomes most critical.
Side Effects Are Usually Brief
The most common side effects of Plan B include nausea, lower abdominal cramping, fatigue, headache, and breast tenderness. These typically resolve within 24 hours. Most people feel completely normal within a day or two.
One important detail: if you vomit within two hours of taking the pill, it may not have been fully absorbed. In that case, you should take another dose. Your next period may arrive a few days early or late, and the flow might be heavier or lighter than usual. If your period is more than a week late, a pregnancy test is a reasonable next step.
Quick Comparison of Your Options
- Plan B (levonorgestrel): Available over the counter, effective up to 72 hours, works best within 24 hours, may be less effective at higher body weights
- Ella (ulipristal acetate): Requires a prescription, effective up to 120 hours, maintains effectiveness better than Plan B throughout the full window
- Copper IUD: Requires a clinic visit for insertion, effective up to 5 days (sometimes longer depending on ovulation timing), over 99 percent effective, doubles as long-term contraception
All three options are more effective the sooner you act. If you’re within 72 hours and can get Plan B right now, take it right now. If you’re past 72 hours, call a provider about ella or a copper IUD. The five-day window still gives you time, but every hour counts.