How Accurate Is Plan B?

Plan B One-Step is the most widely used emergency contraceptive pill in the United States, and its label says it reduces the chance of pregnancy by up to 89 percent when taken as directed. That number is real, but it is also widely misunderstood. The 89 percent is not the same as “89 percent effective.” It refers to a relative reduction in pregnancy risk compared with having unprotected sex with no contraception at all. In absolute terms, roughly 7 out of 100 women who take levonorgestrel emergency contraception after unprotected sex will still become pregnant. The distinction between those two numbers is the single most important thing to understand about how accurate Plan B really is.

What Is Plan B and How Does It Work?

Plan B One-Step is a single 1.5 mg tablet of levonorgestrel, a synthetic form of the hormone progesterone used in many birth control pills. It is available over the counter without a prescription or age restriction in the United States.

Its primary mechanism is delaying or preventing ovulation. Levonorgestrel works by suppressing the luteinizing hormone surge that triggers the release of an egg from the ovary. If ovulation has not yet happened, the drug can postpone it long enough for sperm to die off before an egg is available to fertilize.

What Plan B does not do is equally important. It does not terminate an existing pregnancy. It does not prevent a fertilized egg from implanting in the uterus. Multiple reviews of the evidence have found no support for the idea that levonorgestrel interferes with implantation. It also does not protect against sexually transmitted infections.

This mechanism explains why timing matters so much. Once the luteinizing hormone surge has begun, the drug cannot stop ovulation. If an egg has already been released, Plan B has no way to prevent fertilization. The window in which it can work is narrow, and that window is the main reason its real-world effectiveness is lower than many people assume.

How Accurate Is Plan B Compared to Other Emergency Contraceptives?

There are three main options for emergency contraception in the US, and they are not equally effective. The differences are substantial enough that the choice matters.

The copper intrauterine device is the most effective form of emergency contraception by a wide margin. Research consistently shows it prevents pregnancy in more than 99 percent of cases when inserted within five days of unprotected sex. It also provides ongoing contraception for years afterward. The catch is practical: it requires a office visit, a trained clinician, and a procedure. Many women cannot access one quickly enough for it to be an option.

Ella (ulipristal acetate) is a prescription pill that works similarly to Plan B but remains effective longer, up to 120 hours after unprotected sex. Some evidence indicates it may be more effective than levonorgestrel for women with a higher body mass index, though the research on this point is not entirely settled.

Does Plan B Work If You Are Overweight?

Body weight appears to affect how well levonorgestrel emergency contraception works. This is one of the most debated areas in reproductive health, and the evidence is genuinely mixed.

Some studies have found that levonorgestrel becomes less effective at higher body weights, particularly above roughly 165 to 176 pounds. One widely cited analysis suggested the drug may lose most of its effectiveness in women above a certain weight threshold. Other researchers have challenged those findings, arguing that the data are not strong enough to support a firm cutoff and that the relationship is more gradual than a simple threshold.

What is clear is that the label does not include a weight restriction, and regulatory agencies have not issued one. The American College of Obstetricians and Gynecologists has acknowledged that obesity may reduce the effectiveness of levonorgestrel emergency contraception but has not recommended against its use. For women with higher body weight, the copper IUD or ulipristal acetate may be better options, and some clinicians recommend them for this reason.

This is a case where the honest answer is that the evidence is incomplete. Anyone concerned about weight-related effectiveness should discuss options with a healthcare provider rather than assume the pill will or will not work based on a number.

What Lowers Plan B’s Real-World Accuracy?

The gap between Plan B’s effectiveness in studies and its effectiveness in real life comes down to three factors.

• Timing. Plan B is most effective the sooner it is taken. It is approved for use within 72 hours, but it works best in the first 24 hours. Waiting even a day can reduce how well it works, because the drug’s only real opportunity to act is before ovulation begins.
• Where you are in your cycle. If you are already ovulating or about to ovulate, Plan B cannot help. This is not a failure of the drug — it is a limitation of how it works. Women who track their cycles may have a sense of whether they are in a fertile window, but ovulation timing is unpredictable and can shift from month to month.
• Body weight. As discussed above, higher body weight may reduce effectiveness, though the exact relationship is not fully established.

One point that is often misunderstood: Plan B is not a substitute for ongoing contraception. It is a single high dose of a hormone, and using it repeatedly in the same cycle is not recommended. It is less effective than most regular birth control methods and is not designed to be a primary strategy.

What Should You Do If Plan B Fails?

If you took Plan B and your period is late, the first step is a pregnancy test. Plan B can cause changes to your menstrual cycle, including bleeding that is earlier or later than expected, so a late period does not automatically mean pregnancy. But it is the most reliable signal that something may have changed.

If a pregnancy test is positive, see a healthcare provider. Plan B does not cause birth defects, and there is no evidence that a failed dose of levonorgestrel harms a developing pregnancy. The medication is out of your system within days.

If you are not pregnant and want to avoid future scares, this is a good moment to talk with a provider about a regular contraceptive method. Options like the implant, IUD, pill, patch, ring, and injection are far more effective at preventing pregnancy than emergency contraception. Plan B is a backup, not a plan.

Does Plan B Cause Abortion?

No. Plan B does not cause abortion, and this is one of the most persistent myths about emergency contraception.

The drug works before fertilization, by delaying ovulation. It does not affect an already implanted embryo. Major medical organizations, including the American College of Obstetricians and Gynecologists, classify levonorgestrel emergency contraception as a contraceptive, not an abortifacient. The distinction matters both medically and legally, and it is supported by decades of research.

This is a case where the science is settled. The confusion likely comes from the word “emergency” and from the mistaken belief that the pill works after fertilization. It does not.