Plan B prevents roughly 81% to 90% of pregnancies that would have occurred without it, meaning it fails somewhere between 10% and 19% of the time when taken within 72 hours. That failure rate isn’t fixed, though. It shifts dramatically based on when you take it, how much you weigh, and where you are in your cycle.
Timing Changes the Failure Rate Significantly
The single biggest factor in whether Plan B works is how quickly you take it after unprotected sex. Within the first 24 hours, it’s about 94% effective, giving it a failure rate of roughly 6%. By the time you reach the 72-hour mark, effectiveness drops to around 58%, meaning it fails more than 4 out of 10 times. That’s a massive difference for a 48-hour delay.
Plan B is sometimes used beyond 72 hours, but the evidence for its effectiveness past that window is weak. The pill is approved for use within three days, and every hour you wait chips away at how well it works. If you’re already past the 72-hour mark, a different type of emergency contraception (covered below) remains effective for up to five days.
Why Plan B Can’t Work During Ovulation
Plan B prevents pregnancy by delaying ovulation, the release of an egg from the ovary. If your body hasn’t released an egg yet, the pill pushes that event back long enough for sperm (which survive up to five days inside the body) to die off before they can fertilize anything. It’s a timing trick, not a barrier.
This means Plan B has a critical blind spot: if ovulation has already started, the pill can’t do anything about it. The egg is already out, and sperm may already be waiting. This is probably the most common reason Plan B fails even when taken quickly. Unfortunately, most people don’t know exactly when they ovulate, which makes this limitation hard to plan around. If you track your cycle and suspect you were near ovulation when you had unprotected sex, Plan B is less likely to help.
Weight Has a Major Impact on Effectiveness
Body weight is one of the most underreported reasons Plan B fails. Research shows the pill starts losing effectiveness at around 70 kg (about 154 pounds) and may have essentially no efficacy at 80 kg (about 176 pounds) and above. People with a BMI of 30 or higher who take Plan B have more than four times the risk of pregnancy compared to those with a BMI under 25.
The reason is pharmacological: higher body weight changes how the drug is absorbed and distributed, resulting in lower concentrations of the active ingredient in the bloodstream. This isn’t a minor reduction in effectiveness. For someone weighing over 176 pounds, Plan B may not work meaningfully better than taking nothing at all. If this applies to you, a copper IUD or a prescription emergency contraceptive pill containing a different active ingredient are both stronger options, and both remain effective at higher body weights.
How Plan B Compares to Other Options
A prescription emergency contraceptive pill (sold as Ella) uses a different compound that works somewhat differently. In studies, its pregnancy rate was about 1.2%, compared to 1.2% to 2.1% for Plan B. The gap between the two widens the longer you wait: Ella is significantly more effective than Plan B in the 72-to-120-hour window. It also performs better for people with higher body weight, though it still has limits.
The copper IUD is the most effective form of emergency contraception available. Inserted by a provider within five days of unprotected sex, it prevents more than 99% of pregnancies. It works regardless of body weight and regardless of where you are in your cycle, because it prevents a fertilized egg from implanting. The tradeoff is that it requires a clinic visit and insertion procedure.
Medications That Reduce Plan B’s Effectiveness
Certain drugs speed up the breakdown of Plan B’s active ingredient in your body, leaving less of it available to do its job. These include some anti-seizure medications, anti-HIV drugs, the antibiotics rifampicin and griseofulvin, and the herbal supplement St. John’s wort. If you take any of these regularly, Plan B is more likely to fail, and an alternative method of emergency contraception is worth considering.
How to Know If Plan B Worked
There’s no immediate way to tell. Plan B doesn’t cause a specific symptom that signals success. Some people experience spotting or light bleeding in the days after taking it, and some notice their next period arrives earlier or later than expected, or is heavier or lighter than usual. These are all normal side effects of the hormonal disruption the pill causes, but none of them reliably confirm whether it worked.
The clearest sign that Plan B succeeded is getting your period. Once it arrives, you can be reasonably confident you’re not pregnant. If your period is more than a week late, that’s worth paying attention to. A pregnancy test will give an accurate result about three weeks after the unprotected sex in question. Testing earlier than that can produce a false negative because pregnancy hormone levels may not yet be high enough to detect.