Plan B works by delaying ovulation for roughly five or more days, long enough for sperm (which survive about five days in the reproductive tract) to die off before an egg is released. It does not provide ongoing protection. A single dose covers one episode of unprotected sex, and its ability to prevent pregnancy drops significantly with every day you wait to take it.
How Plan B Actually Works
Plan B contains a large dose of a synthetic hormone that your body recognizes as progesterone. When you take it before ovulation, it suppresses or delays the hormonal surge that triggers egg release. FDA review data show that the pill can push ovulation back by at least five days when the egg-containing follicle is still small (under 18 mm). Once sperm in the reproductive tract die off during that delay, fertilization can’t happen.
This is the only well-supported mechanism. If you’ve already ovulated, Plan B does not meaningfully reduce your chance of pregnancy. The FDA’s own review states that when the pill is taken after ovulation, the pregnancy rate is the same as if nothing had been taken at all. That’s why timing matters so much.
The Effectiveness Window: Hour by Hour
Plan B is most effective the sooner you take it. At 24 hours after unprotected sex, it prevents about 94% of expected pregnancies. By the 72-hour mark, that number drops to roughly 58%. You can still take it between 72 and 120 hours (three to five days), but effectiveness falls further and is not well quantified beyond that point.
The reason for this steep drop is biological. Every hour that passes is an hour closer to when ovulation might naturally occur. If ovulation happens before the drug kicks in, the window is already closed. The pill reaches its peak concentration in your blood within about one to four hours and has a half-life of roughly 27 hours, meaning it clears your system relatively quickly.
It Only Covers One Incident
A common misconception is that Plan B provides a buffer of protection for a few days after you take it. It does not. The pill is designed to address one act of unprotected sex by delaying the ovulation that would have followed. If you have unprotected sex again the next day, or even later that same day, you are not covered by the dose you already took. The FDA labels Plan B explicitly as a backup method, not a routine contraceptive.
If you have another unprotected encounter after taking Plan B, you would need another dose or a different form of emergency contraception. There is no medical restriction on taking a second dose, but relying on it repeatedly is less effective and more expensive than using a regular birth control method.
Body Weight Can Reduce Effectiveness
Clinical guidelines from the UK’s Faculty of Sexual and Reproductive Healthcare indicate that Plan B becomes less effective in people who weigh more than about 155 pounds (70 kg) or have a BMI above 26. At higher body weights, the drug may not reach sufficient levels to reliably suppress ovulation.
If you fall into this range, a different emergency contraceptive pill containing ulipristal acetate (sold as ella) is generally recommended as a more effective option. A copper IUD inserted within five days is the most effective emergency contraception at any weight, preventing over 99% of pregnancies regardless of BMI.
Medications That Can Shorten Protection
Certain drugs speed up how fast your liver breaks down Plan B, potentially reducing its effectiveness. These include some seizure medications (carbamazepine, phenytoin, topiramate, oxcarbazepine, felbamate), the antibiotic rifampin, the antifungal griseofulvin, and the herbal supplement St. John’s wort. Some HIV medications can also alter hormone levels in either direction. If you take any of these regularly, a copper IUD is a more reliable emergency option.
What to Expect Afterward
Because Plan B delivers a large hormonal dose, it commonly shifts your next period. Your period may arrive earlier or later than expected, and the flow may be heavier, lighter, or spottier than usual. Some spotting between taking the pill and your next period is also normal.
Your fertility returns with your next ovulation cycle, which for most people means within a few weeks. The drug does not accumulate in your body or have lasting effects on your ability to conceive. If your period is more than a week late after taking Plan B, a pregnancy test is a reasonable next step, since late periods can also signal that the pill didn’t work.
To stay protected going forward, the most practical approach is to start or resume a regular contraceptive method right away. Hormonal birth control (the pill, patch, ring, implant, or hormonal IUD) can typically be started the same day as or the day after taking Plan B, though it takes several days for most methods to become fully effective on their own.