Plan B stops working within a few days of taking it, and you can get pregnant from sex that happens as soon as one to two days afterward. The active ingredient has a half-life of about 27.5 hours, meaning your body eliminates most of it within two to three days. After that point, you have no residual contraceptive protection.
This surprises many people who assume the pill creates a longer window of safety. It doesn’t. Plan B is designed to prevent one pregnancy from one act of unprotected sex, and it offers zero protection going forward.
How Plan B Works (and Stops Working)
Plan B contains a single dose of a synthetic hormone that delays or prevents ovulation. If your body hasn’t released an egg yet, the pill essentially puts ovulation on pause long enough for sperm (which survive about five days) to die off before they can fertilize anything. If ovulation has already happened, Plan B is far less effective.
Your body processes the drug quickly. With a half-life of roughly 27.5 hours, about half the dose is gone within a day and a quarter remains after two days. By day three or four, the levels in your bloodstream are too low to suppress ovulation. Once your body resumes its normal hormonal pattern, an egg can release on its usual schedule, or even on a slightly shifted one, and you’re fully capable of getting pregnant again.
No Protection for Future Sex
This is the most important practical point: Plan B does not cover any sexual contact that happens after you take it. If you have unprotected sex the next day, or even later the same day, you are not protected by the dose you already took. The University of Iowa Health Care guidelines recommend restarting regular birth control the day after taking emergency contraception and using condoms or abstaining for seven days until that method becomes effective.
If you have another unprotected encounter within those few days, you would technically need another dose of emergency contraception. However, using Plan B repeatedly in the same cycle can make your periods irregular and unpredictable, and it’s significantly less reliable than standard birth control methods.
Your Cycle May Shift Temporarily
Plan B commonly changes the timing of your next period. It may arrive earlier than expected, later than expected, or look different from your norm: heavier, lighter, or more spotty. This is a direct result of the hormonal disruption the pill causes to delay ovulation.
A shifted cycle can make it harder to predict when you’re fertile in the weeks that follow. If your period comes late, you might ovulate later than usual too, which means the days you’d normally consider “safe” could actually be fertile days. Your cycle should return to its regular pattern the following month, but in the immediate cycle after taking Plan B, tracking ovulation by calendar alone is unreliable.
How Effective Plan B Is When You Take It
The pill’s effectiveness drops sharply with every day you wait. Taken within the first 24 hours after unprotected sex, it prevents about 94% of expected pregnancies. By the 72-hour mark, that number falls to around 58%. This steep decline is one reason timing matters so much, and it also illustrates that Plan B is not a guaranteed solution even when taken promptly.
Body weight also plays a role. Research has shown that levonorgestrel-based emergency contraception becomes less reliable for women with a BMI above 26. The hormone appears to be sensitive to weight in a way that reduces its ability to suppress ovulation at higher body masses. If this applies to you, a copper IUD inserted as emergency contraception is the most effective alternative, working regardless of weight and providing ongoing birth control for years afterward.
Returning to Regular Contraception
Because Plan B’s protection evaporates so quickly, what you do in the days after taking it matters more than the pill itself. If you’re on hormonal birth control (the pill, patch, or ring), restart it the next day. You’ll need backup protection like condoms for seven days while the hormones rebuild to effective levels.
If you’re not currently using any method, this is the window where an unplanned pregnancy is most likely to happen. Many people take Plan B during a gap in contraception and then assume the emergency dose gives them a buffer. It doesn’t. Within roughly 48 to 72 hours, your fertility returns to baseline, and depending on where you are in your cycle, ovulation could be just days away.