Plan B (levonorgestrel) works primarily by delaying ovulation, and the delay typically lasts several days, long enough for sperm already in the reproductive tract to die before an egg is released. Sperm can survive inside the body for 3 to 5 days, so the goal of the delay is to push ovulation past that window of viability.
The exact number of days varies from person to person and depends heavily on where you are in your cycle when you take it. Here’s what we know about the timing, effectiveness, and what to expect afterward.
How Plan B Delays Ovulation
Each month, a hormone called luteinizing hormone (LH) surges to trigger ovulation. Plan B contains a synthetic form of progesterone that can suppress this LH surge, which stalls the development and release of an egg. Think of it as pressing pause on your cycle at a critical moment.
This pause doesn’t last a fixed number of hours or days. It depends on how close you were to ovulating when you took the pill. If your body hadn’t yet started ramping up LH, Plan B can delay ovulation long enough for all viable sperm to die off, which takes roughly 3 to 5 days. If the LH surge was already underway or ovulation had already happened, the pill has little to no effect on the process. Studies show that when levonorgestrel is given after ovulation has occurred, pregnancy rates are identical to those seen with a placebo, meaning it doesn’t work after the egg is already released.
Why Timing Matters So Much
Plan B is most effective when taken as soon as possible. Studies show it can be around 94% effective when taken within the first 24 hours after unprotected sex, but that drops to about 58% by 72 hours. This decline isn’t because the drug gets weaker over time. It’s because the closer you get to ovulation, the harder it becomes to suppress the LH surge and delay the process.
If you’re early in your cycle, days away from ovulation, Plan B has plenty of room to work. If you’re a day or two from ovulating, there’s a narrower window where it can still intervene. And if ovulation has already happened, the pill won’t prevent pregnancy because it doesn’t interfere with fertilization or implantation. Its mechanism is specifically about delaying the egg’s release.
The Sperm Survival Factor
The reason the length of the delay matters comes down to sperm. According to the Mayo Clinic, sperm can survive for about 3 to 5 days inside the cervix, uterus, and fallopian tubes. Plan B’s job is to keep ovulation on hold until those sperm are no longer viable.
If the pill delays ovulation by even a few days, the sperm from unprotected sex will have died off by the time the egg is finally released. No viable sperm means no fertilization. This is why earlier use leads to better results: the sooner you take it, the longer the delay before ovulation, and the more time sperm have to lose viability before an egg appears.
How This Affects Your Next Period
Because Plan B disrupts your normal hormonal timeline, your next period will likely shift. It may come earlier or later than expected, and it may feel different: heavier, lighter, or more spotty than usual. These changes are a direct result of the ovulation delay rippling through the rest of your cycle.
If you took Plan B early in your cycle, you may notice a bigger shift in your period’s timing since ovulation was pushed back further, which pushes everything else back too. If you took it closer to when ovulation would have happened naturally, the shift may be smaller. There’s no single number of days to expect because it depends entirely on your individual cycle timing.
If your period hasn’t arrived within three weeks of taking Plan B, take a pregnancy test. A delay of a few days to a week is common and not a sign that something went wrong, but the three-week mark is a reasonable point to check.
What Plan B Does Not Do
Plan B does not prevent a fertilized egg from implanting in the uterus. Research has confirmed that levonorgestrel works by disrupting ovulation, not by interfering with implantation. In studies where the drug was given after fertilization was believed to have already occurred, it had no effect on pregnancy rates compared to a placebo. Its entire mechanism depends on being taken before ovulation, not after.
This also means Plan B is not effective as an ongoing contraceptive method. It intervenes in one specific hormonal event, the LH surge, during one cycle. It doesn’t provide protection for future unprotected sex, even if it occurs within the same cycle.