Plan B is emergency contraception, and it works best the sooner you take it. The main pill sold in the US, levonorgestrel, prevents pregnancy most effectively when taken within 72 hours of unprotected sex, but it is not equally effective across that window. It works mainly by delaying or stopping ovulation. If an egg has already been released, it cannot stop a pregnancy that has already begun.
So the honest answer to “how effective is a Plan B” is: quite effective early, less effective as time passes, and not a guarantee at any point. It is not an abortion pill. It does not end an existing pregnancy. Understanding what it can and cannot do helps you use it correctly and know when another option might be better.
How Does Plan B Actually Work?
Plan B and its generic versions contain levonorgestrel, a synthetic form of the hormone progesterone. The pill works primarily by delaying ovulation. If it holds back the release of an egg until the sperm from that encounter are no longer viable, fertilization cannot happen.
This timing matters more than most people realize. Sperm can survive in the reproductive tract for several days. An egg lives only about 12 to 24 hours after release. Emergency contraception works by keeping those two from meeting.
What levonorgestrel does not do is interrupt an established pregnancy. It does not prevent a fertilized egg from implanting, and it does not cause an implanted embryo to detach. This is the key difference between emergency contraception and the abortion pill, mifepristone. The two are often confused, but they do entirely different things at entirely different stages.
One point worth clarifying: if ovulation has already occurred when you take the pill, it likely will not help for that cycle. That is why speed matters so much. The window is not a countdown to zero effectiveness, but a gradual decline.
How Effective Is a Plan B at Preventing Pregnancy?
Levonorgestrel emergency contraception reduces the chance of pregnancy substantially, but it is not 100 percent effective and its effectiveness drops the longer you wait. Taken within 72 hours, it prevents a significant share of expected pregnancies. Taken within the first 24 hours, it works better than when taken closer to the 72-hour mark.
Rather than quote a single number, it helps to understand the pattern. The pill is most effective in the first day, still effective in the second and third days, and progressively less reliable after that. Some research suggests it retains some benefit up to 120 hours (five days) after unprotected sex, but the evidence for that extended window is weaker than for the first 72 hours.
Two factors lower effectiveness:
- Body weight. Some evidence indicates levonorgestrel works less well for people with a higher body mass index. The exact threshold is debated, and guidance varies. If you weigh more, a different method may be more reliable.
- Timing in your cycle. If you are already close to ovulation, the pill has less opportunity to delay it.
Plan B does not protect against pregnancy for the rest of your cycle. If you have unprotected sex again after taking it, you can still become pregnant. It is a single-use emergency measure, not a method you can rely on going forward.
What About Ella and the Copper IUD?
There are more effective emergency options than Plan B, and knowing about them matters.
Ella (ulipristal acetate) is a prescription emergency contraceptive. It is generally considered more effective than levonorgestrel, especially in the 72-to-120-hour window, and it may work better for people with a higher body weight. It is taken as a single pill. Because it is a different drug, it can be an option when levonorgestrel is less likely to work.
The copper IUD is the most effective form of emergency contraception available. When inserted within five days of unprotected sex, it prevents pregnancy very effectively and then continues to work as ongoing birth control for years. The catch is practical: it requires a visit to a clinician who can place it, and not everyone can access that quickly.
| Method | How Taken | Time Window | Notes |
|---|---|---|---|
| Levonorgestrel (Plan B) | One pill, over the counter | Best within 72 hours | Less effective with higher body weight |
| Ulipristal (Ella) | One pill, prescription | Up to 120 hours | May work better later in the window |
| Copper IUD | Inserted by a clinician | Up to 5 days | Most effective; also long-term birth control |
If you can get to a clinic quickly, the copper IUD or Ella may be worth asking about. If not, Plan B is far better than nothing.
When Should You Take It, and What Are the Side Effects?
Take emergency contraception as soon as possible after unprotected sex. Do not wait to see whether you miss a period. The whole point is to act before ovulation, and that window closes fast.
Plan B is available over the counter without a prescription and without age restriction in the US. You do not need to show ID or talk to a pharmacist to buy it.
Common side effects include:
- Nausea, sometimes with vomiting
- Headache
- Fatigue
- Dizziness
- Breast tenderness
- Bleeding between periods or a change in your next period’s timing
These usually pass within a few days. If you vomit within a few hours of taking the pill, it may not have been absorbed, and you should talk to a pharmacist or clinician about whether to take another dose.
Your next period may come earlier or later than expected. That is normal. But if your period is more than a week late, take a pregnancy test. Emergency contraception lowers the odds of pregnancy; it does not eliminate them.
Why Plan B Is Not the Abortion Pill
This confusion causes real problems, so it is worth stating plainly. Plan B prevents pregnancy. The abortion pill ends an existing pregnancy.
Levonorgestrel works before fertilization by delaying ovulation. Mifepristone, used in medication abortion, works after implantation by blocking the hormone progesterone that sustains a pregnancy. They act at different stages and do different jobs.
This distinction matters for two reasons. First, if you are already pregnant and take Plan B, it will not end the pregnancy. Second, if you need to end a pregnancy, emergency contraception is not the tool for that. You would need to talk to a clinician about abortion options, which are a separate category of care.
Emergency contraception is not a substitute for ongoing birth control, and it is not designed to be used repeatedly as a primary method. It is a backup for when something goes wrong.
What Plan B Cannot Do
Being clear about limits prevents false confidence.
Plan B cannot protect you from sexually transmitted infections. It is not a condom. If you are concerned about STIs, testing is a separate step.
Plan B cannot guarantee you will not get pregnant. No emergency contraceptive is 100 percent effective, and effectiveness falls the longer you wait and the closer you are to ovulation. If your period is late, test.
Plan B cannot be counted on for future protection. Once it has done its job for one encounter, it is spent. If you have unprotected sex again, you can still conceive.
And Plan B does not work if you are already pregnant. It is not a way to end a pregnancy you have already confirmed.
The most reliable approach is to use it fast, understand its limits, and follow up with a pregnancy test if your period does not arrive on schedule. If you are unsure whether it worked or whether you need a different option, a clinician or pharmacist can help you sort it out.
Frequently Asked Questions
How effective is Plan B the morning after?
Plan B is most effective when taken as soon as possible, ideally within the first 24 hours after unprotected sex. Its effectiveness gradually decreases the longer you wait, so taking it right away gives you the best chance.
Can Plan B stop a pregnancy that has already started?
No. Plan B works mainly by delaying ovulation before fertilization happens, so it cannot end a pregnancy that has already begun. If you are already pregnant, it will not work.
Does Plan B work if you are ovulating?
If ovulation has already occurred, Plan B is unlikely to prevent pregnancy for that cycle. This is why timing matters so much and why taking it early is important.
Is Plan B less effective if you weigh more?
Some evidence indicates levonorgestrel works less well at a higher body mass index, though the exact threshold is debated. If you weigh more, a prescription option like Ella or the copper IUD may be more reliable.
