Plan B works primarily by delaying ovulation, the release of an egg from the ovary. Its active ingredient, a synthetic hormone called levonorgestrel, disrupts the hormonal chain reaction your body needs to ovulate. If no egg is released, sperm have nothing to fertilize, and pregnancy can’t occur. The pill contains a single 1.5 mg dose of levonorgestrel, which is the same hormone found in many daily birth control pills but at a much higher concentration.
How It Stops Ovulation
Each month, your body builds toward ovulation through a carefully timed hormonal sequence. A spike in luteinizing hormone (LH) triggers the ovary to release a mature egg. Plan B works by suppressing or delaying that LH spike, pushing ovulation back by at least five days. Since sperm can only survive in the reproductive tract for about five days, this delay is usually enough to prevent any surviving sperm from encountering an egg.
Timing matters enormously here. If you take Plan B before the LH surge has started, it can effectively block ovulation. But if the surge has already begun or ovulation has already occurred, the drug loses this ability. Studies comparing women who took levonorgestrel before versus after the LH surge found that those who took it after had ovulation and conception rates no different from women who took nothing at all. This is the single biggest reason Plan B becomes less effective the longer you wait.
Effects on Cervical Mucus
Levonorgestrel also changes the consistency of cervical mucus, making it thicker and harder for sperm to pass through. Research on levonorgestrel users found that none of their cervical mucus samples allowed good sperm penetration, compared to 77-85% penetration in non-users. While most of this data comes from sustained-release forms of levonorgestrel rather than a single emergency dose, the thickening effect likely plays a supporting role, creating an additional barrier to sperm reaching an egg.
What Plan B Does Not Do
Plan B does not end a pregnancy. The FDA has reviewed the evidence and concluded that levonorgestrel has no direct effect on fertilization or implantation. If a fertilized egg has already implanted in the uterine lining, Plan B will not disrupt it. If you are already pregnant when you take it, the pill will not harm the pregnancy. Its mechanism is preventive: it stops the process before fertilization can happen, not after.
How Quickly It’s Absorbed
After you swallow the tablet, levonorgestrel is absorbed rapidly and reaches its peak concentration in your bloodstream within about 1.5 to 2 hours. From there, your liver metabolizes it with a half-life of roughly 24 hours, meaning half the drug is cleared from your system in about a day. The initial hormonal effects, particularly the suppression of the LH surge, begin within that first couple of hours.
Why Timing Changes Effectiveness
Plan B is around 94% effective when taken within the first 24 hours after unprotected sex. By 72 hours, effectiveness drops to about 58%. This steep decline isn’t because the drug is wearing off. It’s because your body is moving closer to ovulation with every passing hour. The further along you are in that hormonal sequence, the harder it is for levonorgestrel to interrupt it. After 72 hours, the pill is not recommended because there’s too little evidence it still works reliably.
Body Weight and Effectiveness
Your weight can influence how well Plan B works. Pooled data from randomized trials shows a sharp increase in pregnancy rates among women weighing over 70 kg (about 154 pounds) or with a BMI above 26. At higher body weights, the standard 1.5 mg dose may not produce high enough blood levels to reliably suppress the LH surge. Some clinical guidelines suggest that women above these thresholds consider an alternative emergency contraceptive called ulipristal acetate, which maintains its effectiveness across a wider weight range. A copper IUD, inserted within five days, is another option that works regardless of weight.
Medications That Reduce Its Effectiveness
Certain medications speed up how quickly your liver breaks down levonorgestrel, potentially cutting its blood levels in half or more. The most significant interactions include medications used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampin, some HIV medications including efavirenz, and the herbal supplement St. John’s wort. The effect of these drugs on liver enzymes can persist for up to four weeks after you stop taking them, so even recently discontinued medications can be a concern. If you take any of these, the standard Plan B dose may not provide adequate protection.
Effects on Your Next Period
Because Plan B works by disrupting your normal hormonal cycle, it commonly shifts the timing of your next period. Your period may arrive up to a week later than expected. Some women experience it earlier than usual, and others notice heavier or lighter bleeding, spotting between periods, or more intense cramping. These changes are temporary and resolve on their own within one to two cycles. If your period is more than a week late, a pregnancy test is a reasonable next step.