Plan B is most effective within the first 24 hours after unprotected sex, when it prevents pregnancy about 94% of the time. Its effectiveness drops significantly with each passing day, falling to roughly 58% by the 72-hour mark. While the official recommendation is to take it within 72 hours (three days), there is some evidence it offers reduced protection up to five days, and after that, it likely offers no protection at all.
How Effectiveness Changes Hour by Hour
The single most important factor in how well Plan B works is how quickly you take it. At 24 hours, you’re looking at around 94% effectiveness. By 72 hours, that number has dropped to about 58%. That’s a steep decline, and the trend is clear: every hour you wait reduces your chances of preventing pregnancy.
Between 72 and 120 hours (three to five days), the picture gets murkier. A pooled analysis of four World Health Organization trials found no sharp decline in efficacy for levonorgestrel until day five, when it may offer no protection at all. In a study comparing pregnancy rates inside and outside the 72-hour window, women who took emergency contraceptive pills between 72 and 120 hours still had a pregnancy rate (1.8%) significantly lower than what would be expected without any contraception, though higher than the rate for those who took it sooner (0.8%). So there is still some benefit past the three-day mark, but the window is closing fast.
Why Timing Matters So Much
Plan B works by delaying or preventing ovulation, the release of an egg from the ovary. It does this by suppressing the hormonal surge that triggers egg release. This is why timing is everything: the pill needs to act before that surge begins.
If ovulation has already happened, Plan B is essentially ineffective. FDA review data confirm that when levonorgestrel is taken after ovulation, pregnancy rates are the same as if no emergency contraception were taken at all. The pill doesn’t prevent a fertilized egg from implanting. It simply keeps the egg from being released in the first place. This means its effectiveness depends not just on how many hours have passed since sex, but also on where you are in your menstrual cycle.
Body Weight Can Reduce Effectiveness
Research has found that Plan B becomes less reliable for women with a BMI above 26, which corresponds to roughly 155 to 175 pounds depending on height. This was an unexpected finding when it first emerged, and it has important practical implications. If your weight falls in this range or higher, a prescription alternative called ella (ulipristal acetate) may be a better option. Ella is approved for use up to 120 hours (five full days) after unprotected sex and has shown superior efficacy compared to Plan B, particularly for later use and for people with higher body weight.
Ella as a Late-Window Alternative
If you’re past the 24-hour window or concerned about Plan B’s declining effectiveness, ella works through a different mechanism that remains effective longer. It’s FDA-approved for use up to 120 hours after unprotected sex and maintains more consistent effectiveness throughout that entire window, rather than declining steeply like Plan B does. The tradeoff is that ella requires a prescription, though some telehealth services can provide one quickly.
Ella works by blocking the hormonal receptor that triggers ovulation, which means it can still delay egg release even when the hormonal surge has already started. Plan B cannot do this. For someone approaching or past the 72-hour mark, ella is the stronger choice among pill-based options.
Taking Plan B More Than Once
You can safely take Plan B more than once in the same menstrual cycle. The American College of Obstetricians and Gynecologists endorses repeated use within the same cycle if needed, and studies have found no increased risk of serious side effects from doing so. You may experience some menstrual irregularities, like spotting or a shift in your period timing, but these changes are typically mild.
One important detail: because Plan B only works by delaying ovulation, each act of unprotected sex requires its own dose. Taking it once doesn’t provide ongoing protection for the rest of your cycle. If you have unprotected sex again after taking Plan B, you’d need to take it again.
What to Expect Afterward
After taking Plan B, your next period may arrive earlier or later than usual. It might also be heavier or lighter than normal. These shifts happen because the hormone in Plan B temporarily disrupts your cycle’s natural timing. If your period is more than a week late, a pregnancy test is a reasonable next step.
Some medications can also reduce Plan B’s effectiveness. Anti-seizure medications like phenytoin, carbamazepine, and topiramate, along with the antibiotic rifampin (used for tuberculosis), can speed up how quickly your body breaks down the hormone in Plan B, potentially making it less effective.
The Bottom Line on Timing
The practical answer: take Plan B as soon as possible. Within 24 hours gives you the best odds at around 94%. By 72 hours, you’re down to about 58%. Between three and five days, there’s still some reduced benefit, but the protection is significantly weaker. After five days, levonorgestrel likely offers no protection at all. If you’re past 72 hours, weigh heavier than about 155 pounds, or want the strongest late-window option available, ella is worth pursuing through a prescription.