How Long Can You Wait To Take A Plan B? Facts

Emergency contraception works best the sooner you take it, and how long you can wait depends entirely on which product you use. Levonorgestrel pills (sold as Plan B One-Step and generic versions) are approved for use up to 72 hours after unprotected sex. The prescription pill ulipristal acetate (ella) is approved up to 120 hours. The copper IUD is the most effective option and can be placed up to 5 days after unprotected sex.

Those numbers are the outer limits set by regulators and clinical guidelines. They are not equally effective across the whole window. Taking any of these options earlier generally works better than taking the same option later.

How Long Can You Wait To Take A Plan B?

Levonorgestrel emergency contraception is approved for use within 72 hours of unprotected sex. That is the number on the label and in the prescribing information.

The catch is that effectiveness is not flat across those 72 hours. The drug works mainly by delaying or preventing ovulation. If ovulation has already happened, it will not stop a pregnancy that is already underway. The closer you get to ovulation, the less room the drug has to work. Research consistently shows that levonorgestrel becomes less effective as more time passes between sex and taking the pill.

This is why “you have 72 hours” can be misleading. Yes, the window is 72 hours. But a pill taken at hour 12 is working with more biological room than one taken at hour 68. If you can take it sooner, take it sooner.

Body weight also matters. Evidence indicates that levonorgestrel emergency contraception becomes less effective at higher body weights, and some guidance suggests it may not work well above a certain threshold. If you weigh more, ask a pharmacist or clinician about other options. This is a real limitation, not a rumor.

Is There an Emergency Contraception Option That Works Longer Than 72 Hours?

Yes. Ulipristal acetate, sold under the brand name ella, is approved for use up to 120 hours after unprotected sex. That is 5 days instead of 3.

Ulipristal acetate is a prescription-only pill. It works by delaying ovulation, similar in principle to levonorgestrel, but it stays effective closer to the point of ovulation. Some studies suggest it remains somewhat more effective than levonorgestrel in the later part of the window, though the evidence is not perfectly settled and effectiveness still declines with time.

The copper IUD is the third option and the most effective of the three. It can be inserted up to 5 days after unprotected sex. Unlike the pills, the copper IUD prevents fertilization and also works after ovulation by changing the environment in the uterus so a fertilized egg cannot implant. It is also the only emergency option that provides ongoing contraception afterward. The trade-off is that it requires a visit to a clinician and a procedure.

Which Emergency Contraception Should You Choose and When?

The best choice depends on how much time has passed, your body weight, and what you can access quickly.

• Within 72 hours: Levonorgestrel is widely available over the counter without a prescription or ID requirement. It is the fastest option to get.
• 72 to 120 hours: Ulipristal acetate is approved in this window but requires a prescription. The copper IUD also works in this window if you can get it placed.
• Higher body weight: The copper IUD is not affected by body weight. Ulipristal acetate may be a better pill option than levonorgestrel, but ask a clinician about your specific situation.
• Wanting ongoing protection: The copper IUD provides years of contraception after insertion.

One practical point: levonorgestrel and ulipristal acetate should not be taken together, and if you take ulipristal acetate, starting a hormonal birth control pill right away can reduce its effectiveness. A clinician can walk you through the timing.

Does Emergency Contraception Work After Ovulation?

This is where the biology gets specific, and it matters for understanding the time limits.

Levonorgestrel and ulipristal acetate both work primarily by delaying or preventing ovulation. If the egg has already been released, these pills do not have a mechanism to stop a pregnancy that has already begun. The sperm may already have met the egg. Once that happens, the pill cannot undo it.

The copper IUD is different. It works both before and after ovulation. Copper ions in the uterus are toxic to sperm and also prevent a fertilized egg from implanting in the uterine lining. That is why it remains effective up to 5 days after sex, even if ovulation happened during that window.

This distinction explains why timing is not just about the clock on the wall. It is about where you are in your cycle. If you ovulated the day after unprotected sex, a levonorgestrel pill taken on day 3 may be too late to help. If you had not ovulated yet, the same pill taken on day 3 may still work.

Most people do not know exactly when they ovulate. That uncertainty is one more reason to take emergency contraception as soon as possible rather than using the full window.

What Happens If You Take Plan B Too Late?

If you take levonorgestrel after 72 hours, it is not approved to work and there is no reliable evidence that it prevents pregnancy in that window. If you take ulipristal acetate after 120 hours, the same applies.

If the window has passed, the copper IUD may still be an option if you are within 5 days of the unprotected sex. Beyond that, the options change.

If you have had unprotected sex and the emergency contraception window has closed, a clinician can discuss other steps. A pregnancy test is generally accurate about 2 to 3 weeks after sex, or around the time of a missed period. If you are concerned about pregnancy, testing and talking to a healthcare provider is the next step. Do not wait for symptoms to appear, because early pregnancy often has no symptoms at all.

Emergency contraception is not meant to be a regular method. It is for emergencies. If you find yourself needing it often, a clinician can help you find a birth control method that fits your life better.

How Effective Is Emergency Contraception, Really?

Effectiveness depends on the method and the timing, and the numbers are not as simple as a single percentage.

Levonorgestrel is most effective when taken in the first 24 hours and becomes less effective with each passing day. Ulipristal acetate stays effective longer and is generally considered at least as effective as levonorgestrel, possibly more so in the later part of the window. The copper IUD is the most effective emergency method overall, with very low failure rates when placed within 5 days.

No emergency contraception method is 100 percent effective. None of them protect against sexually transmitted infections. If you are at risk for an STI, testing is a separate and important step.

One more thing worth knowing: emergency contraception is not an abortion pill. It does not end an existing pregnancy. It works by preventing pregnancy from happening in the first place, and only if taken before the relevant biological steps have occurred.