Plan B works by delaying ovulation, typically pushing it back by several days. The exact delay varies depending on where you are in your menstrual cycle when you take it, but the downstream effect on your period can be up to one week. The closer you are to ovulating when you take Plan B, the less likely it is to successfully postpone egg release.
How Plan B Delays Ovulation
Plan B contains a synthetic hormone called levonorgestrel that suppresses the hormonal surge your body needs to release an egg. If that surge hasn’t happened yet, Plan B can pause the process long enough for any sperm in your reproductive tract to die off (sperm survive up to five days). This is the pill’s only meaningful way of preventing pregnancy: it stops the egg from showing up.
The delay isn’t a fixed number of days for everyone. It depends on how far along your body is in preparing to ovulate. If you’re early in your cycle and ovulation is still days away, Plan B has more hormonal runway to work with and can reliably hold things off. If your body is already ramping up toward ovulation, the window to intervene is much narrower.
What Happens If You’ve Already Ovulated
Plan B does not work if ovulation has already occurred. Once your body has released an egg, the pill has no mechanism to prevent fertilization or implantation that makes a clinical difference. This is why timing matters so much, and why the pill is more effective the sooner you take it after unprotected sex.
The tricky part is that most people don’t know exactly when they ovulate. Ovulation doesn’t come with obvious symptoms for many people, and cycle tracking apps are estimates at best. If you’re unsure where you are in your cycle, taking Plan B is still worth it. You may not have ovulated yet, and if that’s the case, the pill can still do its job.
How This Affects Your Next Period
Because Plan B delays ovulation, it also delays everything that follows, including your period. Your next period may come up to a week later than expected. Some people experience it slightly earlier. Both are normal responses to the hormonal disruption.
If your period doesn’t arrive within three weeks of taking Plan B, that’s the point to take a pregnancy test. Light spotting in the days after taking the pill is also common and isn’t the same as a period.
Effectiveness Drops Quickly With Time
Plan B is around 94% effective when taken within the first 24 hours after unprotected sex. By 72 hours (three days), effectiveness drops to about 58%. The CDC notes that levonorgestrel emergency contraception can be taken up to five days after unprotected sex, but the closer you get to that outer limit, the less likely it is to prevent pregnancy.
This declining effectiveness reflects the biological reality: every hour that passes is an hour closer to potential ovulation. The sooner you take the pill, the more likely it is to delay ovulation before sperm can reach an egg.
Body Weight Can Reduce Effectiveness
Research published in the journal Contraception found a significant drop in Plan B’s effectiveness at higher body weights. Women weighing between 75 and 85 kilograms (roughly 165 to 187 pounds) had a pregnancy rate of about 6.4%, compared to 1.4% for those weighing 65 to 75 kilograms. The study found that pregnancy risk climbed steeply starting around 70 to 75 kilograms, and at higher weights, pregnancy rates were similar to what you’d expect without any contraception at all.
If you weigh more than about 165 pounds, a different type of emergency contraception (sold under the brand name ella) may be more effective. That option requires a prescription but works through a different mechanism and maintains effectiveness across a wider weight range. A copper IUD, inserted within five days, is the most effective emergency contraception regardless of weight.
Taking Plan B More Than Once Per Cycle
If you’ve had unprotected sex more than once in the same cycle, you can take Plan B again. The American College of Obstetricians and Gynecologists supports repeated use within the same menstrual cycle when needed, and studies haven’t found increased health risks from doing so. You may notice more menstrual irregularities, like unexpected spotting or a longer delay before your next period, but these changes are typically mild.
Repeated use can make your cycle harder to predict for that month, which in turn makes it harder to know if the pill worked. If you’re relying on emergency contraception more than occasionally, a regular contraceptive method will be both more effective and more predictable. After taking Plan B, you can start or resume regular contraception immediately, but you’ll need to use condoms or abstain for seven days while it takes effect.