How Long Is Plan B Good For? Timeline

Plan B One-Step is effective for up to 72 hours after unprotected sex, and the research shows it works better the sooner you take it. The single-tablet form is approved by the FDA for use within that 72-hour window. There is no evidence it provides any protection if taken beyond that window, and it will not end a pregnancy that has already implanted.

That 72-hour limit is the part most people know. What gets less attention is how sharply effectiveness drops across those three days, what the manufacturer’s labeling actually says, and why timing matters so much for how this drug works.

How Long Is Plan B Good For After Unprotected Sex?

Plan B One-Step is labeled for use within 72 hours (three days) of unprotected sex or a contraceptive failure, such as a condom breaking. The FDA approved it on that basis, and 72 hours is the outer limit of the approved window.

Within that window, timing changes the odds considerably. The drug works best in the first 24 hours. Effectiveness is lower on day two and lower still on day three. This pattern is consistent across the clinical literature on levonorgestrel emergency contraception. If you are deciding whether to take it, sooner is better, and waiting to see whether your period arrives is not a useful strategy.

One point that causes confusion: the 72-hour clock starts from the act of unprotected sex, not from when you ovulate or expect your period. If there were multiple acts of unprotected sex over several days, the window runs from the most recent one.

What Does the Manufacturer’s Label Actually Say?

The label for Plan B One-Step states the product should be taken as soon as possible within 72 hours after unprotected sex. That is the approved indication, and it is what the manufacturer can legally claim based on the data submitted for approval.

You may have heard that Plan B works for up to five days. That claim belongs to a different product. Ella (ulipristal acetate) is a prescription emergency contraceptive approved for use within 120 hours, or five days. It is not the same drug as Plan B, and the two are not interchangeable in terms of window or mechanism.

Some clinicians will discuss off-label use of levonorgestrel beyond 72 hours with patients. That is a clinical judgment call, not an approved use, and the evidence for benefit in that window is limited. If you are past 72 hours, the more reliable option is to contact a healthcare provider about ulipristal acetate or a copper IUD, both of which have evidence supporting use beyond 72 hours.

Why Does Timing Matter So Much?

Plan B does not work the way many people assume. It is not an abortion pill, and it does not stop a fertilized egg from implanting. The evidence indicates its main job is to delay or prevent ovulation.

Levonorgestrel, the active ingredient, is a progestin. When taken before the luteinizing hormone (LH) surge that triggers the release of an egg, it can suppress or delay that surge. No egg is released, so there is nothing for sperm to fertilize. That is the mechanism.

This explains the timing curve. Once ovulation has begun, delaying it is no longer possible, and the drug has nothing left to do. If ovulation has already happened and fertilization has occurred, Plan B will not interrupt the process. It also will not prevent a fertilized egg from implanting.

There is a common misconception that Plan B causes an abortion. It does not. The medical consensus, supported by the drug’s mechanism of action, is that it works before fertilization by preventing the egg from being released in the first place.

This mechanism also explains why Plan B is less effective in certain situations. If the LH surge has already started, the drug may not be able to stop it. That is why it does not work as well later in the cycle, even within the 72-hour window.

How Effective Is Plan B Within the 72-Hour Window?

Effectiveness depends heavily on when in the cycle the drug is taken and how soon after sex it is taken. The overall picture from clinical research is that levonorgestrel emergency contraception reduces the risk of pregnancy compared with no intervention, but it is not a guarantee.

It is not a 100% effective method. No emergency contraceptive is. If you take Plan B and still become pregnant, it is not because you did something wrong. It means the drug did not prevent ovulation in time, or ovulation had already occurred.

Body weight may affect how well levonorgestrel works. Some research suggests that levonorgestrel emergency contraception may be less effective in people with higher body weight, though the evidence is not entirely consistent and the exact threshold is debated. The FDA has not added a weight-based restriction to the label. If you weigh more than about 165 pounds and are considering emergency contraception, it is worth discussing options with a healthcare provider, including ulipristal acetate or a copper IUD.

What If You Take Plan B After 72 Hours?

If more than 72 hours have passed, Plan B is not approved for use and there is no reliable evidence that it will prevent pregnancy in that window. Taking it late is not dangerous, but it is unlikely to help.

The better options past 72 hours are ulipristal acetate (Ella), which is approved for up to 120 hours, and the copper IUD, which can be inserted as emergency contraception for up to five days after unprotected sex and is the most effective option available. Both require a prescription or a visit to a healthcare provider.

If you are past 72 hours and cannot access either option, contact a healthcare provider or a reproductive health clinic as soon as possible. Do not wait to see whether your period comes before seeking care.

What Are the Side Effects and What Should You Expect?

The most common side effects of Plan B are headache, nausea, abdominal pain, fatigue, and changes to your next period. The period may come earlier or later than expected, and it may be heavier or lighter. These effects are usually temporary.

If your period is more than a week late after taking Plan B, take a pregnancy test. Plan B can shift your cycle, so a late period is not automatically a sign of pregnancy, but it is worth checking.

Plan B is not intended for regular use as a primary contraceptive. It is less effective than ongoing methods and is meant for emergencies. If you find yourself needing it repeatedly, that is a signal to talk with a provider about a method that fits your life better.

One more point: Plan B does not protect against sexually transmitted infections. Condoms remain the only method that reduces that risk.

How Does Plan B Compare to Other Emergency Options?

The copper IUD is the most effective emergency contraceptive option, but it requires a visit and a procedure. Ulipristal acetate has a longer window than levonorgestrel and may be more effective, particularly for people with higher body weight, though it is available by prescription only.

If you are within 72 hours and can access Plan B quickly, it is a reasonable first option. If you are closer to the 72-hour mark or beyond it, ask about ulipristal acetate or a copper IUD.