Plan B is not 100% effective. Studies show a pregnancy rate of 1.2% to 2.1% among people who take it, and its effectiveness drops significantly the longer you wait. Taken within the first 24 hours after unprotected sex, Plan B is about 94% effective. By the 72-hour mark, that number falls to roughly 58%.
How Effective Plan B Actually Is
Plan B works by delaying or stopping the release of an egg from the ovary. If your body hasn’t ovulated yet, the pill can pause that process so there’s no egg available to be fertilized. The FDA has concluded that Plan B has no effect on what happens after ovulation, meaning it does not prevent a fertilized egg from implanting and does not end an existing pregnancy.
That mechanism is exactly why Plan B isn’t a guarantee. Its ability to work depends on where you are in your menstrual cycle at the time you take it. If ovulation has already happened or is actively underway, the pill has very little to offer. Research published in the journal Contraception found that women who took Plan B at or after the hormonal surge that triggers ovulation had conception rates nearly identical to women who took a placebo. In one study of 45 women who had already ovulated, 8 became pregnant compared to 8.7 expected pregnancies without any intervention.
This is the core limitation: Plan B can only prevent pregnancy if it can still block ovulation. If that window has closed, the pill essentially doesn’t work.
Why Timing Matters So Much
The clock starts the moment you have unprotected sex, and every hour counts. At 24 hours, Plan B prevents about 94% of expected pregnancies. At 72 hours, effectiveness drops to around 58%. The pill is approved for use up to 72 hours after intercourse, but taking it as soon as possible gives you the best odds by far.
The reason for the steep decline is straightforward. The closer you get to ovulation, the less time the pill has to suppress the hormonal signals that trigger egg release. If you happen to have sex a day or two before you would naturally ovulate and then wait two more days to take Plan B, the drug may arrive too late to stop anything.
Body Weight Affects How Well It Works
Your weight can significantly reduce Plan B’s effectiveness. Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced morning-after pill failure four times as often as those with a BMI under 25. The reason appears to be pharmacological: blood levels of the active drug were 50% lower in people with a BMI of 30 after taking a standard dose. The drug simply never reaches a high enough concentration to reliably block ovulation.
Doubling the dose doesn’t solve the problem. The same research team tested whether taking two Plan B pills would compensate for the lower drug levels in higher-BMI individuals, and it did not improve pregnancy prevention. If you weigh more than 165 pounds, a different type of emergency contraception is likely a better option.
More Effective Alternatives
If Plan B’s limitations concern you, two alternatives offer stronger protection. The first is ella, a prescription emergency contraceptive pill that uses a different active ingredient. Ella is more effective than Plan B at every time point within the five-day window, and it maintains better efficacy for people who weigh more than 165 pounds. Unlike Plan B, ella can still work closer to ovulation because it actively suppresses the hormonal surge rather than just delaying it.
The most effective option is a copper, Mirena, or Liletta IUD inserted within five days of unprotected sex. IUDs used as emergency contraception work just as well on day five as they do on day one, and they prevent more than 99% of pregnancies. They also double as long-term birth control once placed. The tradeoff is that insertion requires a clinic visit, which may not be practical on a short timeline.
How to Know If Plan B Worked
Plan B commonly shifts the timing of your next period. It may come a few days early, a few days late, or be heavier or lighter than usual. These changes are normal side effects of the hormone dose and don’t tell you much about whether the pill succeeded.
The reliable signal is whether your period arrives at all. If you haven’t gotten your period within three weeks of taking Plan B, take a pregnancy test. That three-week window gives your body enough time to either resume its normal cycle or produce enough pregnancy hormone for a test to detect.