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 steadily from there: to roughly 85-90% at 72 hours, and it continues to decline through the five-day (120-hour) mark. The sooner you take it, the better it works.
How Effectiveness Changes Hour by Hour
Plan B’s clock starts ticking the moment you have unprotected sex. At the 24-hour mark, it’s roughly 94% effective. By 72 hours (three days), effectiveness falls to around 58-90%, depending on the study and how it measures success. The drug does retain some ability to prevent pregnancy between 72 and 120 hours, but the evidence for that extended window is weaker, and another emergency contraceptive option may be a better choice if you’re past the three-day mark.
The key takeaway is that every hour matters. Taking Plan B the morning after is meaningfully more protective than taking it the evening after, which is meaningfully more protective than waiting another day. If you have it available, take it as soon as possible rather than waiting.
Why Timing Matters So Much
Plan B works by preventing or delaying ovulation, the release of an egg from the ovary. It does not terminate a pregnancy or affect a fertilized egg that has already implanted. This is important because it means Plan B only works if ovulation hasn’t happened yet.
Research shows that if you take Plan B before your body has started the hormonal surge that triggers ovulation (called the LH surge), it can successfully block that surge and stop the egg from being released. But if ovulation has already occurred, Plan B provides essentially no protection. In one study, women who took it on the day of ovulation or afterward became pregnant at the same rate as women who used no contraception at all.
This explains why the pill becomes less effective with each passing hour. The longer you wait, the closer you may be to ovulating, and once that window closes, the drug can’t help.
Plan B Only Covers Past Intercourse
A single dose of Plan B protects against pregnancy from the sex you already had. It does not act as ongoing contraception for the rest of your cycle. If you have unprotected sex again after taking Plan B, even the next day, you’re at risk of pregnancy from that new encounter. You would need to take another dose to address the new exposure.
Body Weight Can Reduce Effectiveness
Plan B may not work as well for everyone. Health Canada has noted that it may be less effective in women over 165 pounds and could be ineffective in women over 176 pounds. Those thresholds roughly correspond to a BMI of 25 and 30, respectively. The evidence isn’t definitive enough to say there’s zero benefit at higher weights, and medical organizations have said women with a higher BMI shouldn’t be discouraged from using it. But the reduction in effectiveness is real.
For women with a BMI over 30, a prescription alternative called ella (ulipristal acetate) has shown better results. The risk of unintended pregnancy with ella in that group is about 2.5%, compared to roughly 6% with Plan B. A copper IUD inserted within five days is another highly effective option regardless of weight.
How Ella Compares Beyond 72 Hours
If you’re past the 72-hour window, ella is the stronger option. Within the first three days, ella and Plan B perform similarly. But between 72 and 120 hours, ella maintains its effectiveness while Plan B’s drops considerably. Ella works by blocking the body’s progesterone receptors, which means it can delay ovulation even after the hormonal surge has started, something Plan B cannot do.
Ella requires a prescription in the United States, while Plan B is available over the counter with no age restriction. If you’re within 24 hours and can get Plan B immediately, it’s a strong choice. If more time has passed, especially beyond 72 hours, ella or a copper IUD is worth pursuing.
Taking Plan B More Than Once
There is no medical limit on how many times you can take Plan B in a single menstrual cycle. Frequent use is not associated with long-term side effects, and it does not cause infertility. If you have unprotected sex on two separate occasions, you can take a dose for each one.
That said, taking an extra dose for the same encounter won’t make it more effective. One dose per exposure is the correct approach. And if you’ve already taken ella since your last period, you should not take Plan B, because the two drugs can interfere with each other. Plan B is meant as a backup, not a primary method of contraception, simply because it’s less effective than regular birth control options like an IUD, implant, or daily pill.