Plan B works by delaying ovulation long enough for sperm to die before an egg is released, but the delay isn’t a fixed number of days. Its effectiveness depends almost entirely on where you are in your menstrual cycle when you take it. When taken before the hormonal surge that triggers ovulation, levonorgestrel (the active ingredient in Plan B) can suppress or postpone egg release for several days, potentially long enough to outlast the 3- to 5-day window that sperm can survive in the reproductive tract. Once that surge has already started, the drug has little to no effect on ovulation timing.
How Plan B Actually Delays Ovulation
Plan B’s primary job is to blunt or delay the spike in luteinizing hormone (LH) that tells the ovary to release an egg. This LH surge normally peaks one or more days before ovulation occurs. If you take Plan B before that surge begins, the drug can push ovulation back long enough that any sperm from unprotected sex are no longer viable by the time the egg finally appears. Sperm survive about 3 to 5 days in the uterus and fallopian tubes, so the goal is to delay egg release beyond that window.
The drug reaches peak levels in your bloodstream quickly, typically within about 2 to 2.5 hours of swallowing the pill, with a long half-life of 30 to 46 hours. That means it stays active in your system for a couple of days, giving it a sustained window to interfere with the hormonal signals that trigger ovulation.
The Timing That Makes or Breaks It
This is the critical piece most people don’t realize: Plan B’s ability to delay ovulation drops sharply depending on when in your cycle you take it. Research shows that when taken around two to three days before expected ovulation, levonorgestrel caused no measurable delay in when the egg was released. By that point, the LH surge is already underway or about to begin, and the drug can’t reliably stop the process.
If taken earlier in the follicular phase, well before the LH surge starts, the drug is far more effective at suppressing ovulation. But if the surge has already peaked, Plan B has no apparent impact on pregnancy risk. Some researchers estimate that because Plan B can’t work once the surge is underway, its real-world effectiveness in the final two days before ovulation may be less than 40%.
This is why timing matters so much. Taking it sooner gives it a better chance of catching you before that hormonal tipping point.
Why “Take It as Soon as Possible” Is Real Advice
The effectiveness numbers back this up clearly. When taken within 72 hours of unprotected sex, Plan B prevents 87% to 90% of expected pregnancies. Between 72 and 120 hours, that drops to 72% to 87%. The decline isn’t because the drug wears off. It’s because the longer you wait, the more likely your body has already initiated the ovulation process that Plan B can’t reverse.
Every hour of delay slightly increases the chance that your LH surge has already started. That’s why the standard recommendation is to take it as quickly as possible, ideally within the first 24 hours.
What Plan B Does Not Do
A common concern is whether Plan B prevents a fertilized egg from implanting in the uterus. A review of 33 studies found that women who took levonorgestrel at or after the LH surge had similar conception rates as women who took a placebo. Nine out of ten studies in that review found no difference in how receptive the uterine lining was to implantation. The evidence strongly supports that Plan B works before fertilization, not after. If ovulation has already happened and fertilization occurs, the drug does not appear to interfere with pregnancy.
Body Weight and Effectiveness
There’s ongoing debate about how well Plan B works at higher body weights. Health Canada has stated that levonorgestrel may be less effective in women over 165 pounds and potentially ineffective over 176 pounds. However, the European Medicines Agency reviewed the same data and concluded the evidence was too limited to draw firm conclusions. Major medical organizations, including the Society of Obstetricians and Gynaecologists of Canada, recommend that women with a BMI over 30 should not be discouraged from using levonorgestrel if it’s their most accessible option. If weight is a concern, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraceptive regardless of body size.
How It Affects Your Next Period
Plan B commonly shifts the timing of your next period, and the direction depends on when in your cycle you took it. In a study of over 200 women, about 21% had their cycle shortened by two or more days, while 24% had it lengthened by two or more days. Women who took it before ovulation tended to get their period about a day early. Women who took it after ovulation saw their period arrive roughly two days late.
These shifts are generally mild. If your period is more than five days late, a pregnancy test is worth taking, not because Plan B failed, but because the delay could indicate it was taken too late in the cycle to prevent ovulation.
Can You Take It More Than Once Per Cycle
Yes. The American College of Obstetricians and Gynecologists endorses that levonorgestrel emergency contraception can be used repeatedly in the same menstrual cycle if needed. Studies have not found an increased risk of serious side effects with repeated use in a single cycle, though you may notice more menstrual irregularity. Each dose works independently, so a second unprotected encounter later in the same cycle warrants another dose if you’re not using ongoing contraception. That said, if you’re needing it frequently, a regular contraceptive method will be far more reliable and less disruptive to your cycle.