Plan B reaches peak levels in your bloodstream within about 1.5 to 2 hours of swallowing the pill. But “working” in this case doesn’t mean it needs time to build up like an antibiotic. The pill delivers a single, large dose of a synthetic hormone that acts on your reproductive system almost immediately after absorption. The critical factor isn’t how fast it kicks in, but how soon after unprotected sex you take it.
How Plan B Prevents Pregnancy
Plan B works by delaying or blocking ovulation, the release of an egg from the ovary. Your body normally triggers ovulation through a hormonal signal called the LH surge. The large dose of synthetic hormone in Plan B suppresses that surge, so the egg never releases. Without an egg, sperm have nothing to fertilize.
This is also why timing matters so much. If your body has already begun ovulating before you take the pill, Plan B has no way to pull the egg back. Research published in the journal Contraception found that women who took the pill at or after the LH surge had conception rates similar to women who used no emergency contraception at all. In other words, once ovulation is underway, Plan B provides little to no protection.
How Quickly It’s Absorbed
According to FDA review data, the active ingredient reaches its maximum concentration in the blood in roughly 1 to 2 hours, with an average of about 1.6 hours. Absorption is rapid because the pill contains a straightforward oral hormone, not a slow-release formulation. Once absorbed, it begins interacting with the hormonal signals that control ovulation right away.
If you vomit within 2 hours of taking Plan B, your body may not have absorbed enough of the dose. In that case, you need to take another pill. Vomiting after the 2-hour mark means the drug has already entered your system and you don’t need to redose.
Why Every Hour Counts
Plan B’s effectiveness drops sharply with each day you wait. Studies show it’s about 94% effective when taken within the first 24 hours after unprotected sex. By the 72-hour mark (3 days), that number falls to roughly 58%. The pill is not recommended beyond 72 hours for levonorgestrel-based products, though some prescribers note limited benefit up to 5 days.
The reason for the decline is straightforward: the closer you get to ovulation, the less likely the pill can stop it. If you had unprotected sex on a Monday and ovulation was going to happen Wednesday, taking the pill Monday night gives it time to suppress that process. Waiting until Wednesday morning may be too late.
Body Weight Affects How Well It Works
Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. Blood levels of the active hormone were about 50% lower in individuals with a BMI of 30 after a standard dose, meaning the drug never reached the concentration needed to reliably block ovulation. Doubling the dose did not solve this problem in a clinical trial of 70 participants.
If your weight is above 176 pounds or your BMI is 30 or higher, a copper IUD inserted within 5 days of unprotected sex is the most effective emergency contraception option. Another prescription alternative, which uses a different active ingredient, may also work better in this weight range, though it still has limitations.
Medications That Reduce Its Effectiveness
Certain drugs speed up how quickly your liver breaks down Plan B’s active ingredient, leaving less of it circulating in your blood. The UK’s medicines safety agency specifically warns about:
- Epilepsy medications such as carbamazepine, phenytoin, and barbiturates
- Tuberculosis drugs such as rifampicin
- HIV medications such as efavirenz, which cuts blood levels of the hormone by about 50%
- St. John’s wort, an herbal supplement commonly used for mood support
If you take any of these regularly, Plan B may not reach effective levels in your system regardless of how quickly you take it. A copper IUD is a more reliable alternative in this situation.
Side Effects and What to Expect Afterward
Common side effects include nausea, headache, dizziness, and breast tenderness. These typically show up within a few hours of taking the pill, roughly matching the absorption timeline, and resolve within a day or two. Nausea is the most frequently reported, which is why the vomiting guidance exists.
Your next period may come earlier or later than expected. Some people experience spotting or lighter bleeding before their regular period arrives. If your period is more than a week late, a pregnancy test is a reasonable next step, since it could mean the pill didn’t prevent ovulation in time.
The Bottom Line on Timing
Plan B is absorbed and active in your body within about 2 hours. But its ability to prevent pregnancy depends entirely on whether it reaches your system before ovulation begins. Taking it within 12 hours of unprotected sex gives you the best odds. Every hour of delay narrows the window, and by 72 hours, effectiveness has dropped by more than a third. The pill works fast once you swallow it. The real variable is how fast you take it.