Plan B works best within the first 24 hours after unprotected sex, when it’s about 94% effective. Its effectiveness drops significantly over time, falling to roughly 58% by the 72-hour mark. After 72 hours, it still offers some protection but considerably less, and by five days (120 hours) it’s essentially at its limit. The short answer: the sooner you take it, the better it works, and waiting past three days means you should seriously consider a more effective alternative.
How Effectiveness Changes Hour by Hour
Plan B contains levonorgestrel, and its ability to prevent pregnancy declines in a steep curve. Within the first 24 hours, it reduces the risk of pregnancy by about 94%. By 72 hours, that number drops to around 58%. A meta-analysis reviewed by the CDC found that pregnancy rates remained low when levonorgestrel was taken within four days but increased noticeably at the four-to-five-day mark.
The FDA label says 72 hours is the window, but CDC guidelines from 2024 note that emergency contraceptive pills can be taken up to five days after unprotected sex. The catch is that levonorgestrel becomes significantly less reliable in that extended window compared to other options. If you’re past the three-day mark, Plan B isn’t your best choice anymore.
Why Timing Matters So Much
Plan B works primarily by delaying or preventing ovulation, the release of an egg from the ovary. Your body triggers ovulation through a hormone surge, and levonorgestrel can interfere with that surge if it hasn’t happened yet. That’s why timing is everything: the pill needs to act before your body releases the egg.
Once ovulation has already occurred, Plan B loses its ability to prevent pregnancy. Research confirms that when levonorgestrel is given on the day of ovulation or afterward, the number of pregnancies that occurred matched the number that would have been expected without any intervention at all. In other words, it had zero effect. Even when taken one or two days before ovulation, studies found that follicular rupture (the egg being released) still occurred in roughly 80 to 92% of cases. The closer you are to ovulation, the less likely Plan B can intervene in time, regardless of how quickly you take it after sex.
Better Options After 72 Hours
If you’re past the three-day window, a prescription pill called ella (ulipristal acetate) is the stronger oral option. It remains effective for a full five days after unprotected sex, and the gap between ella and Plan B grows wider as time passes. Between 72 and 120 hours, ella is substantially more effective. Both the CDC and clinical evidence support ella as the preferred pill when you’re beyond day three.
The most effective emergency contraception at any point within five days is a copper IUD. It can be placed by a healthcare provider within five days of unprotected sex, and in some cases even beyond five days, as long as it’s placed within five days of estimated ovulation. Unlike pills, which try to prevent ovulation from happening, the copper IUD works by creating an environment that prevents fertilization and implantation, making it reliable even if you’ve already ovulated. It also doubles as long-term birth control afterward.
Body Weight Can Reduce Effectiveness
Plan B may be less effective for people who weigh more than 165 pounds, and some regulators have suggested it may be ineffective above 176 pounds. That doesn’t mean it’s completely useless at higher weights, but the reduction in effectiveness is real. For people with a BMI over 30, the risk of unintended pregnancy with levonorgestrel is roughly 6%, compared to about 2.5% with ella. If your weight is a factor, ella or a copper IUD are stronger choices regardless of timing.
What to Expect Afterward
Plan B commonly causes nausea, fatigue, headache, and changes to your next period. Your period may come earlier or later than expected, and the flow might be heavier or lighter than usual. These effects are temporary and typically resolve within a few days to a week.
Because Plan B can shift your cycle, you can’t rely on a late period alone to tell you whether it worked. Take a pregnancy test three weeks after taking the pill for an accurate result. If your period is more than a week late and you haven’t tested yet, that’s a good signal to pick up a test.
The Practical Takeaway
Every hour counts. At 24 hours, Plan B is highly effective. At 72 hours, it’s working at a little over half its potential. Past 72 hours, it still offers marginal protection, but ella or a copper IUD are meaningfully better options. Past five days, no emergency contraceptive pill is recommended. If you’re unsure where you are in your cycle or how close you might be to ovulation, acting fast gives Plan B the best chance of working, because once the egg is released, the pill can’t undo that.