Plan B One-Step is about 89% effective at preventing pregnancy when taken within 72 hours of unprotected sex, but that number tells only part of the story. Its real-world effectiveness depends heavily on how quickly you take it, where you are in your menstrual cycle, and your body weight. Under the best circumstances, it can reduce your chance of pregnancy from roughly 8% down to about 1%. Under less ideal circumstances, it may barely work at all.
How Plan B Actually Works
Plan B contains a single dose of levonorgestrel, a synthetic hormone that prevents or delays ovulation. It blocks the hormonal surge that triggers your ovary to release an egg. If that egg never releases, sperm have nothing to fertilize, and pregnancy can’t happen.
This mechanism has an important implication: Plan B only works if you haven’t ovulated yet. If ovulation has already occurred, the pill will not prevent pregnancy. It also does not interrupt a pregnancy that has already started or harm a developing embryo. Despite persistent misconceptions, research has not found evidence that Plan B prevents implantation of a fertilized egg.
Effectiveness by the Hour
Timing is the single biggest factor determining whether Plan B works. The pill is most effective when taken as soon as possible after unprotected sex, and its effectiveness drops sharply with every passing hour.
Studies show Plan B is around 94% effective when taken within the first 24 hours. By the 72-hour mark, effectiveness drops to roughly 58%. One analysis found that every 12 hours of delay cuts efficacy by about 50%. The label says you can take it up to 72 hours after sex, and while it may still offer some protection beyond that window, the data past three days is not encouraging for levonorgestrel specifically.
In practical terms, the difference between taking Plan B at hour 6 and hour 60 is enormous. If you’re considering it, the best time to take it is right now.
How Body Weight Affects Effectiveness
This is one of the most important and least-discussed limitations of Plan B. Research shows that levonorgestrel becomes significantly less effective as body weight increases.
People with a BMI over 30 (the clinical threshold for obesity) have more than four times the risk of pregnancy compared to those in a normal BMI range when using levonorgestrel emergency contraception. Those with a BMI between 25 and 30 are about twice as likely to experience a failure. One reanalysis of clinical trial data found that Plan B appears to hit a ceiling of efficacy at about 154 pounds (70 kg) and showed essentially no emergency contraceptive effect for people weighing 176 pounds (80 kg) or more.
These are significant thresholds. The average American woman weighs about 170 pounds, meaning Plan B may offer reduced protection for a large portion of the people who need it. If your weight is above these thresholds, a different type of emergency contraception is likely a better choice.
How Plan B Compares to Ella
Ella (ulipristal acetate) is a prescription emergency contraceptive pill that works through a different mechanism and generally outperforms Plan B, especially as time passes. In a large randomized trial of over 2,200 women, the pregnancy rate within 72 hours was 1.8% for Ella compared to 2.6% for Plan B. A broader meta-analysis confirmed Ella reduced the odds of pregnancy by about 42% compared to levonorgestrel when both were taken within 72 hours.
The difference becomes even more dramatic after the three-day mark. Ella remains effective for up to five days (120 hours) after unprotected sex. In one trial, all three pregnancies that occurred in the 72-to-120-hour window happened in the levonorgestrel group. None occurred in the Ella group. So if more than two days have passed, Ella is the clearly superior option.
Ella also appears to maintain its effectiveness better across a wider weight range, though it requires a prescription, which can be a barrier when time is critical.
The Copper IUD as Emergency Contraception
The most effective form of emergency contraception isn’t a pill at all. A copper IUD inserted within five days of unprotected sex reduces the risk of pregnancy by more than 99%. It works regardless of body weight and regardless of where you are in your cycle, because it creates an environment in the uterus that is hostile to both sperm and fertilized eggs. It also then provides ongoing contraception for up to 10 years. The obvious downside is that it requires a clinic visit and insertion by a healthcare provider, which may not be feasible on short notice.
When Plan B Won’t Help
Because Plan B works by blocking ovulation, its effectiveness depends entirely on where you are in your menstrual cycle at the time of unprotected sex. If you had sex during or just after ovulation, the pill has little to offer. Unfortunately, most people don’t know exactly when they ovulate, which makes this a gamble.
The hormonal surge that Plan B blocks (called the LH surge) is the trigger for ovulation. If that surge has already peaked and the egg has been released, levonorgestrel can’t reverse the process. This is part of why the overall effectiveness numbers aren’t higher. The 89% figure is an average across all cycle days. For women who happened to take it before ovulation, the protection is quite high. For those who took it after, it may have done nothing.
What to Expect After Taking It
Plan B is a large dose of hormone, and it commonly causes side effects. Nausea is the most frequent, affecting roughly one in four users. You may also experience headaches, fatigue, breast tenderness, or dizziness. Your next period may arrive earlier or later than expected, and it may be heavier or lighter than usual. These effects are temporary and typically resolve within a few days.
If your period is more than a week late after taking Plan B, take a pregnancy test. The pill is not 100% effective under any circumstances, and a late period could indicate it didn’t work. A positive test after taking Plan B does not mean the pill harmed the pregnancy. It simply means ovulation had already occurred or was too imminent for the drug to stop it.
Choosing the Right Emergency Contraception
Plan B is widely available over the counter without a prescription or age restriction, which makes it the most accessible option. For someone who weighs under 155 pounds and can take it within 24 hours, it’s a reasonable and effective choice. Outside those parameters, its reliability drops considerably.
If you weigh more than 155 pounds, if more than 48 hours have passed, or if you want the highest possible protection, Ella or a copper IUD will give you better odds. Ella requires a prescription but can be obtained through telehealth services. A copper IUD requires a provider visit but offers near-perfect protection regardless of timing or weight, as long as it’s placed within five days.