Plan B works best the sooner you take it. The U.S. Food and Drug Administration has approved levonorgestrel emergency contraception, sold as Plan B One-Step and generic versions, for use within 72 hours (3 days) after unprotected sex. The label says 72 hours, but that is not a cutoff line where the drug suddenly stops working. It is the window the manufacturer studied and the FDA reviewed.
If you are asking because you need it right now, the practical answer is this: take it as soon as you can get it. Do not wait for a “better” day to take it. If more than 72 hours have passed, you may still have options, and a copper IUD is one of them. Read on for how the timing actually works.
When Can You Take Plan B?
You can take Plan B any time after unprotected sex, starting immediately. The label window is 72 hours, and the evidence for effectiveness is strongest within that window. Many clinicians will still offer it up to 120 hours (5 days), though the evidence for benefit becomes weaker as time passes.
The reason timing matters is biological. Plan B works mainly by delaying or preventing ovulation. If an egg has already been released and a sperm has already met it, Plan B will not undo that. The drug is not a “morning after” abortion pill. It does not end a pregnancy that has already started.
This is a point where a lot of confusion lives. Plan B and mifepristone (the medication used in medication abortion) are different drugs with different jobs. Plan B is emergency contraception. It prevents pregnancy. It does not terminate one.
How Does the 72-Hour Window Actually Work?
Levonorgestrel, the active ingredient in Plan B, works by slowing the release of the egg from the ovary. If it can keep the egg from being released during the days when sperm are still alive in the reproductive tract, pregnancy is prevented.
Sperm can survive in the reproductive tract for up to about 5 days. This is why the timing of ovulation relative to intercourse matters so much. If you had sex a few days before ovulation, there may already be sperm waiting. If Plan B delays ovulation past the point where those sperm are no longer viable, it can still work.
That is also why Plan B is less effective when ovulation is already underway or has just happened. The drug has nothing left to delay. This is not a failure of the drug. It is a limitation of how it works.
Effectiveness numbers are often quoted, but they vary by study and by how close to ovulation the sex occurred. The general pattern is consistent: the sooner you take it, the better it works. The exact percentage depends on where you were in your cycle.
What If More Than 72 Hours Have Passed?
You still have options. The most effective one is the copper IUD, which can be inserted up to 5 days after unprotected sex. It works by preventing fertilization and is the most effective form of emergency contraception available.
Levonorgestrel pills are sometimes offered beyond 72 hours, but the evidence for benefit drops off as time passes. Some clinicians will prescribe them up to 120 hours. The honest position is that the pill is less reliable the later you take it, and the copper IUD is more reliable in that window.
If you are past 5 days, emergency contraception is no longer an option. A pregnancy test taken at the right time is the next step. Home pregnancy tests are generally reliable about a week after a missed period. If you are unsure about timing, a healthcare provider can help you sort it out.
Does Plan B Work Differently Based on Body Weight?
There is real evidence that levonorgestrel emergency contraception may be less effective at higher body weights. Some studies suggest reduced effectiveness above a certain BMI, though the exact threshold is debated and the data are not uniform.
This does not mean Plan B does not work at all for people with higher body weights. It means the evidence suggests it may work less well. The copper IUD does not appear to have this limitation, which is one reason it is often recommended as the most reliable option.
If you have a higher BMI and are considering emergency contraception, this is a reasonable thing to bring up with a pharmacist or clinician. Ulipristal acetate, sold as ella, is another prescription emergency contraceptive that may retain effectiveness at higher body weights, though the evidence is also not perfectly clear.
What Affects How Well Plan B Works?
Several things shape whether Plan B will do its job. None of them are things you can control after the fact, but they help explain why results vary.
- Timing: The single biggest factor. Earlier is better.
- Where you are in your cycle: If ovulation has already happened, Plan B cannot stop it.
- Body weight: Higher body weight may reduce effectiveness.
- Other medications: Some drugs, including certain seizure medications and rifampin, can reduce how well levonorgestrel works.
- How many times you had unprotected sex: Plan B does not protect against pregnancy from sex that happens after you take it.
That last point matters and is often missed. Plan B is for the sex that already happened. It is not a substitute for ongoing birth control. If you have unprotected sex again after taking it, you are not protected.
What Should You Expect After Taking Plan B?
Plan B can cause side effects, and they are usually temporary. The most common ones are nausea, headache, fatigue, and changes in when your next period arrives. Some people spot between periods. Others find their period comes a few days early or late.
If you vomit within a few hours of taking Plan B, the dose may not have been absorbed. Call a pharmacist or clinician to ask whether you need to take another dose. Do not guess.
Your next period may be different from usual. If it is more than a week late, take a pregnancy test. Plan B is not 100 percent effective, and a late period is the signal to check.
Where Can You Get Plan B and Do You Need a Prescription?
Plan B One-Step and its generic equivalents are available over the counter with no prescription and no age restriction in the United States. You can buy them at pharmacies, some grocery stores, and online. You do not need to show ID.
Ella (ulipristal acetate) is a different emergency contraceptive that does require a prescription. It is taken within 120 hours of unprotected sex. Because it is a prescription drug, you will need to see a clinician or use a telehealth service.
The copper IUD must be inserted by a healthcare provider. It is the most effective emergency option and also works as ongoing birth control for years afterward.
Frequently Asked Questions
Can you take Plan B the morning after?
Yes. You can take it immediately after unprotected sex, and there is no minimum wait time. The sooner you take it, the more effective it is likely to be.
Is Plan B still effective at 72 hours?
The FDA-approved window is 72 hours, and the evidence for effectiveness is strongest within that time. It may still be offered up to 120 hours, but the evidence for benefit weakens as time passes.
Does Plan B work if you have already ovulated?
Plan B works mainly by delaying ovulation, so if an egg has already been released, it cannot stop that. This is why timing relative to your cycle matters so much.
How late can you take Plan B?
The label says 72 hours, though some clinicians will prescribe it up to 120 hours. The copper IUD is the most effective option within the 5-day window.
