Does Plan B Always Work? What Actually Affects Its Success

TL;DR

• Plan B is about 95% effective within 24 hours, drops to roughly 61% at 48 to 72 hours, and is significantly less reliable after that.
• Plan B does not work if you have already ovulated; it prevents pregnancy by delaying ovulation, not by stopping implantation or ending an existing pregnancy.
• Body weight affects effectiveness: Plan B may be less reliable for people over 165 pounds, and alternatives like ella or a copper IUD may be better options.

How Does Plan B Actually Work?

Plan B One-Step contains 1.5 mg of levonorgestrel, a synthetic progestin hormone. It works primarily by delaying or preventing ovulation, which is release of an egg from ovary. If there is no egg available, sperm cannot fertilize anything, and pregnancy does not occur.

This is most important thing to understand about Plan B: it prevents pregnancy before it starts. It is not abortion pill. It does not terminate an existing pregnancy. If a fertilized egg has already implanted in uterus, Plan B will not affect it.

Plan B may also thicken cervical mucus, making it harder for sperm to reach an egg, and it may alter uterine lining. But primary mechanism, and one that determines whether it works or fails, is whether it successfully stops or delays ovulation.

When Is Plan B Most Likely to Work?

Timing is single biggest factor in Plan B's effectiveness. The sooner you take it after unprotected sex, better it works.

Within 24 hours, Plan B is approximately 95% effective. This is its peak performance window. If you can take it within first 12 hours, even better.

Between 24 and 48 hours, effectiveness drops to around 85%.

Between 48 and 72 hours, effectiveness drops further to roughly 61%.

After 72 hours, Plan B can still be taken up to 120 hours (5 days) after unprotected sex, but its effectiveness continues to decline significantly. Taking it on day 4 or 5 is better than nothing, but it is far less reliable than taking it within first day.

This is why having Plan B on hand before you need it matters. If you can take it immediately rather than spending hours getting to a pharmacy, you are working within most effective window.

The Office on Women's Health, part of U.S. Department of Health and Human Services, provides clear guidance on emergency contraception options and their timing windows.

OWH Emergency Contraception Information

When Is Plan B Most Likely to Fail?

The most common reason Plan B fails is that ovulation has already happened. If you have already ovulated before taking Plan B, drug cannot do its primary job of stopping egg release because egg is already out. In this scenario, sperm can still reach and fertilize egg, and pregnancy can occur despite taking pill.

A 2010 study of women aged 18 to 38 at a family planning clinic found that among 35 women who took levonorgestrel on day of ovulation or afterward, six became pregnant. That is a meaningful failure rate and demonstrates that Plan B is not effective once ovulation has occurred.

The challenge is that most people do not know exactly when they ovulate. Ovulation typically happens around day 14 of a 28-day cycle, but cycles vary widely. Stress, illness, travel, and other factors can shift ovulation earlier or later than expected. If you had unprotected sex close to your expected ovulation date, Plan B may not provide adequate protection.

If you are trying to understand your pregnancy risk based on where you were in your cycle, this guide covers relevant details: Pregnancy Risk After Unprotected Sex Near Menstruation

Does Body Weight Affect How Well Plan B Works?

This is a real concern, though data is not entirely settled. Research has shown that Plan B appears to be less effective in people who weigh more than 165 pounds (75 kg). One study found higher pregnancy rates among women with a BMI over 25 after taking levonorgestrel emergency contraception. For people with a BMI over 30, effectiveness may be substantially reduced.

That said, Plan B still lowered pregnancy rates across all weight categories in available studies. It was not completely ineffective in larger-bodied individuals, just less reliable. The manufacturer does not list a specific weight limit, and there are no restrictions on purchasing Plan B based on body weight.

If you weigh over 165 pounds, prescription emergency contraceptive ella (ulipristal acetate) may be a more effective option. Ella works up to 5 days after unprotected sex and has been shown to maintain better effectiveness at higher body weights, up to about 195 pounds.

For people over 195 pounds, a copper IUD inserted within 5 days is most effective form of emergency contraception regardless of weight. It is over 99% effective and also provides ongoing contraception for up to 10 years.

What About Drug Interactions?

Certain medications can reduce Plan B's effectiveness. The most notable are enzyme-inducing drugs that speed up metabolism of levonorgestrel in liver, causing it to be cleared from your body faster. These include certain anti-seizure medications (like phenytoin, carbamazepine, and phenobarbital), tuberculosis drug rifampin, and herbal supplement St. John's wort.

If you are taking any of these, let your provider know. They may recommend ella or a copper IUD instead, as these alternatives are less affected by drug interactions.

If you vomit within 2 hours of taking Plan B, dose may not have been fully absorbed. In this case, you should take another dose as soon as possible.

For a fuller understanding of what can interact with levonorgestrel, this resource covers key considerations: Levonorgestrel Interactions

How Do You Know If Plan B Worked?

The honest answer is that you will not know for sure until your next period arrives or you take a pregnancy test. There is no immediate way to confirm whether Plan B successfully prevented pregnancy.

After taking Plan B, your next period may come a few days early or a few days late. It may be heavier or lighter than usual. Some people experience spotting between periods. These are all normal effects of levonorgestrel and do not tell you whether pill worked.

If your period is more than 7 days later than expected, take a home pregnancy test. The most accurate results come from testing at time of your expected period or later. Testing too early can produce a false negative because hCG levels (the hormone pregnancy tests detect) may not be high enough yet. If your period is on time or close to it, that is a good sign that Plan B worked.

What Are Your Alternatives?

If Plan B is not best option for your situation, there are two other forms of emergency contraception to consider.

Ella (ulipristal acetate) is a prescription emergency contraceptive that works up to 5 days after unprotected sex and maintains stronger effectiveness throughout that window compared to Plan B. It is also more effective at higher body weights. However, it requires a prescription.

The copper IUD (Paragard) is most effective emergency contraceptive available. When inserted within 5 days of unprotected sex, it prevents over 99% of pregnancies. It works by creating an environment that is toxic to sperm and prevents fertilization and implantation. It is not affected by body weight, is not affected by where you are in your cycle, and provides ongoing contraception for up to 10 years. The downside is that it requires a provider visit for insertion.

Conclusion

Plan B is a powerful tool for preventing pregnancy after unprotected sex, but it does not always work. Its effectiveness depends heavily on timing, ovulation status, and body weight. Taking it as soon as possible, ideally within 24 hours, gives you best chance of success. If you have already ovulated, Plan B is unlikely to prevent pregnancy. If you weigh over 165 pounds, ella or a copper IUD may be more reliable alternatives. No form of emergency contraception is 100% effective, which is why consistent, ongoing birth control remains most reliable way to prevent unplanned pregnancy.