Plan B is an emergency contraceptive pill, and it works well — but not perfectly, and not in every situation. Taken as directed, it reduces the chance of pregnancy after unprotected sex, but its effectiveness depends heavily on when in your cycle you take it and how soon after sex you take it. It is not a substitute for regular birth control, and it will not end an existing pregnancy.
How Well Does Plan B Work?
Plan B works primarily by delaying ovulation. If an egg has already been released, the pill does not stop a pregnancy from happening. That single fact explains most of the variation in how well it performs.
Timing matters more than almost anything else. The sooner you take it, the better it works. According to the U.S. Food and Drug Administration, levonorgestrel emergency contraception — the active ingredient in Plan B — reduces the chance of pregnancy by about 89% when taken within 72 hours of unprotected sex. That means roughly 7 out of every 8 women who would have become pregnant without it will not become pregnant.
But that 89% is an average across the whole 72-hour window. It is not the same at hour 12 as it is at hour 70. The effectiveness drops as time passes. This is why the label says “as soon as possible” rather than just “within 72 hours.”
There is a second limitation that gets less attention. Plan B works by delaying ovulation. If you have already ovulated — meaning an egg has been released and is waiting in the fallopian tube — the pill cannot undo that. If sperm are already present, fertilization can still occur. This is not a flaw in the drug. It is simply how it works. No emergency contraceptive pill currently available prevents implantation of a fertilized egg, despite what many people believe.
Does Plan B Work If You Are Already Ovulating?
No. If ovulation has already occurred, Plan B will not prevent pregnancy. This is one of the most important things to understand about how the pill works.
The active ingredient, levonorgestrel, is a synthetic form of progesterone. It works mainly by suppressing the surge of luteinizing hormone that triggers the release of an egg from the ovary. If that surge has already happened and the egg has been released, the window has closed. The pill has nothing left to delay.
Some people assume emergency contraception works like a very early abortion. It does not. Plan B does not affect an already fertilized egg, and it does not prevent a fertilized egg from implanting in the uterus. Research has not found evidence that levonorgestrel interferes with implantation. This distinction matters for both accuracy and for people who hold particular beliefs about when life begins.
The practical takeaway: if you are mid-cycle and close to ovulation, Plan B may not help. This is why some clinicians recommend a different option — a copper IUD — for people who want the most reliable emergency contraception.
How Does Plan B Compare to Other Emergency Contraception?
The copper IUD is the most effective form of emergency contraception available. It works by preventing fertilization and, if fertilization has already occurred, by preventing implantation. It is inserted by a healthcare provider within five days of unprotected sex. It can reduce the risk of pregnancy by more than 99% when used in that window.
Ella (ulipristal acetate) is a prescription emergency contraceptive pill. It is a selective progesterone receptor modulator that can delay ovulation even when the luteinizing hormone surge has already begun — a point at which levonorgestrel is no longer effective. Some studies suggest Ella is somewhat more effective than levonorgestrel, particularly in the later days of the 120-hour window. Ella requires a prescription in the United States.
Plan B is available over the counter without a prescription and without age restrictions. That accessibility is a major advantage. It is also less expensive than an IUD insertion and does not require a clinic visit.
Here is a direct comparison of the main options:
| Method | How It Works | Time Window | Effectiveness | Prescription Needed |
|---|---|---|---|---|
| Levonorgestrel (Plan B) | Delays ovulation | Up to 72 hours (best within 24) | Reduces pregnancy risk by about 89% when taken within 72 hours | No |
| Ulipristal (Ella) | Delays ovulation, may work after LH surge begins | Up to 120 hours | Some studies suggest slightly more effective than levonorgestrel | Yes |
| Copper IUD | Prevents fertilization and implantation | Up to 5 days | More than 99% effective | No, but requires insertion |
Effectiveness figures for Plan B and the copper IUD come from FDA labeling and clinical guidelines. The comparison between Ella and levonorgestrel is based on studies that have shown a modest advantage for Ella, though the evidence is not uniform across all studies.
What Makes Plan B Less Effective?
Body weight appears to affect how well Plan B works. The FDA has noted that levonorgestrel emergency contraception may be less effective in women with a higher body mass index. The exact threshold is debated. Some research points to reduced effectiveness above a BMI of 26, while other analyses suggest the drop is more pronounced above 30.
The evidence here is not settled. The FDA added a note to Plan B labeling about this in 2016, but the agency did not set a specific cutoff. The American College of Obstetricians and Gynecologists has acknowledged that obesity may reduce effectiveness but has not issued a firm BMI threshold for when to choose a different method.
What is clearer: if you have a higher BMI and need emergency contraception, the copper IUD is the most reliable option regardless of weight. Ella may also be a better choice than Plan B for some women with higher body weight, though the data are limited.
Other factors that reduce effectiveness:
- Time since unprotected sex. The longer you wait, the less well it works.
- Ovulation timing. If you have already ovulated, it will not work.
- Certain medications. Drugs that induce liver enzymes — such as rifampin, some seizure medications, and St. John’s wort — can speed up the breakdown of levonorgestrel and reduce its effectiveness.
- Repeat unprotected sex. Plan B does not provide ongoing protection. If you have unprotected sex again after taking it, you are not protected.
What Are the Side Effects of Plan B?
The most common side effects are nausea, headache, fatigue, and changes in your next menstrual period. These usually resolve within a few days. Vomiting within two to three hours of taking the pill can reduce absorption and may require a repeat dose — check with a pharmacist or healthcare provider if this happens.
Plan B may cause your next period to come earlier or later than expected. If your period is more than a week late after taking it, consider taking a pregnancy test. The pill is not 100% effective, and a late period could mean it did not work.
Plan B is not known to cause birth defects if taken early in an unrecognized pregnancy. Research has not found an increased risk of congenital anomalies from levonorgestrel exposure. However, it is not intended for use during pregnancy, and it will not harm or end an existing pregnancy.
Some people experience breast tenderness, dizziness, or spotting between periods. These are typically mild and temporary. If you have severe abdominal pain, seek medical attention — this can be a sign of an ectopic pregnancy, which is rare but serious.
Can Plan B Be Used as Regular Birth Control?
No. Plan B is for emergencies, not routine use. It is less effective than most regular birth control methods, and it does not protect against sexually transmitted infections.
Using Plan B repeatedly is not dangerous in a direct toxic sense, but it is not designed for that purpose. It may cause irregular bleeding and cycle disruption. More importantly, relying on it means you are accepting a higher risk of pregnancy than you would with a method like the pill, an IUD, or an implant.
If you find yourself needing emergency contraception often, that is a signal to talk with a healthcare provider about a regular method that fits your life. Many options are highly effective and do not require daily attention.
How Should You Take Plan B for Best Results?
Take it as soon as possible after unprotected sex. Do not wait to see if your period comes. Do not wait until the next day if you can take it now. The one-pill version of Plan B is a single dose. The two-pill version, if you have it, can be taken as two pills at once or one pill twice, 12 hours apart — both are effective, but taking both at once is simpler and avoids the risk of forgetting the second dose.
You can take Plan B at any point in your cycle. You do not need a pregnancy test first, though if you have reason to think you might already be pregnant, taking Plan B will not harm the pregnancy but it also will not end it.
If you vomit within two to three hours of taking the pill, call your pharmacist or healthcare provider. You may need to take another dose.
After taking Plan B, use a barrier method like condoms until your next period. Your cycle may be irregular, and you cannot rely on Plan B for ongoing protection.
What Plan B Does Not Do
Plan B does not cause an abortion. It does not prevent a fertilized egg from implanting. It does not protect against HIV or other sexually transmitted infections. It does not work if you have already ovulated. It does not provide protection for future acts of intercourse.
Understanding these limits is not a reason to avoid Plan B. It is a reason to use it correctly — quickly, and with realistic expectations. For many people, it is a safe and accessible backup option. For others, especially those with higher body weight or those who may have already ovulated, a different method may be more reliable.
If you are unsure whether Plan B is the right choice for your situation, a pharmacist can answer questions without a prescription or appointment. Most pharmacies stock it on the shelf, and no ID is required.
Frequently Asked Questions
How effective is Plan B after 72 hours?
Plan B is not approved for use beyond 72 hours, and its effectiveness drops significantly after that window. If more than 72 hours have passed, a copper IUD inserted within five days is the most effective option.
Can Plan B work if I have already ovulated?
No. Plan B works by delaying ovulation, so it cannot prevent pregnancy if an egg has already been released. This is why timing in your cycle matters as much as how quickly you take it.
Does Plan B work if I am overweight?
Plan B may be less effective in women with a higher body mass index, though the exact BMI threshold is debated. The copper IUD is the most reliable emergency option regardless of weight.
How late can I take Plan B?
Plan B is approved for use up to 72 hours after unprotected sex, but it works best when taken as soon as possible. Some research suggests it may have some effect up to 120 hours, but it is not approved for that window and is less effective the longer you wait.
