Plan B fails roughly 1 to 2 out of every 100 times when taken within 72 hours, but that number is misleading on its own. The real chance it won’t work depends heavily on how quickly you take it, where you are in your menstrual cycle, and how much you weigh. Under some circumstances, Plan B can be nearly as reliable as regular birth control. Under others, it may barely work at all.
How Timing Changes the Odds
The single biggest factor in whether Plan B works is how many hours have passed since unprotected sex. Taken within the first 24 hours, levonorgestrel (the active ingredient in Plan B) is about 94% effective. That means roughly 6 out of 100 people who would have become pregnant will still become pregnant. Wait until the 72-hour mark, and effectiveness drops to around 58%, meaning it fails more often than it succeeds among those who were at risk of pregnancy.
That steep decline matters. Every hour you wait reduces your protection, so “morning-after pill” is a bit of a misnomer. Taking it the same night is meaningfully better than taking it the next morning, and taking it the next morning is meaningfully better than waiting another day.
Why It Doesn’t Work After Ovulation
Plan B works by delaying ovulation, the release of an egg from the ovary. If you take it before your body has started the hormonal surge that triggers ovulation, the pill can pause that process long enough for sperm (which survive about five days) to die off before an egg ever appears. But if ovulation has already happened or is actively underway, Plan B has little to no ability to prevent pregnancy. It does not stop a fertilized egg from implanting.
This is why the same pill can be highly effective for one person and completely ineffective for another, even when both take it at the same time after sex. What matters is not just the clock, but where you are in your cycle. If you had unprotected sex close to ovulation, your risk of pregnancy is higher to begin with, and Plan B is least likely to help at exactly that moment.
Body Weight Makes a Significant Difference
Research consistently shows that Plan B becomes less effective as body weight increases. The decline begins at around 70 kg (about 154 lbs), and studies suggest essentially no efficacy for people weighing 80 kg (about 176 lbs) or more. People with a BMI of 30 or higher who take Plan B are more than four times as likely to become pregnant compared to those with a BMI under 25.
The reason is straightforward: the standard 1.5 mg dose produces blood levels of the drug that are roughly 50% lower in people with higher body weight. The drug simply doesn’t reach a high enough concentration to reliably delay ovulation. Doubling the dose has been shown to correct this difference in blood levels, but a double dose is not currently part of the standard over-the-counter product.
If you weigh more than 155 lbs and need emergency contraception, a different option will likely be more reliable. Ella (ulipristal acetate) maintains better effectiveness at higher weights, and a copper IUD is the most effective option regardless of weight.
How Plan B Compares to Other Options
Plan B is the most accessible emergency contraceptive because it’s available without a prescription, but it’s not the most effective one. Here’s how the options stack up:
- Copper IUD: A pregnancy rate of 0.09% when placed within five days of unprotected sex. That’s roughly 1 in 1,000, making it by far the most reliable option. It requires a clinic visit for insertion.
- Hormonal IUD (52 mg levonorgestrel): A failure rate of about 0.5% in clinical trials when placed within five days. Also requires a provider.
- Ella (ulipristal acetate): Pregnancy rates of 0.9% to 1.8% within 120 hours (five days). Up to 98% effective in the first 24 hours and still about 85% effective at the five-day mark. Requires a prescription in the U.S.
- Plan B (levonorgestrel): Pregnancy rates of 1.2% to 2.1% overall, but with the steep time-dependent decline described above. Available over the counter.
In head-to-head trials, Ella reduced the odds of pregnancy by 65% compared to Plan B when both were taken in the first 24 hours, and by 42% when taken within 72 hours. Ella also maintains its effectiveness over a longer window, while Plan B’s reliability drops sharply after the first day.
Vomiting and Medications That Interfere
If you vomit within three hours of taking Plan B, your body may not have absorbed enough of the drug. In that case, you likely need another dose. Certain medications also reduce Plan B’s effectiveness by speeding up how quickly your liver breaks down the hormone. These include some drugs used to treat epilepsy, HIV, and tuberculosis, as well as St. John’s wort. Activated charcoal supplements can also interfere with absorption. Common antibiotics like amoxicillin, doxycycline, and ciprofloxacin do not affect how the pill works.
What to Do After Taking It
Plan B commonly shifts your next period by a few days in either direction. Some people experience spotting, heavier bleeding, or nausea in the days following the dose. If your period is more than one week late, take a pregnancy test. If you’re unsure when your period is due, a pregnancy test taken at least 21 days after unprotected sex will give a reliable result. Testing earlier than that can produce a false negative because pregnancy hormone levels may not yet be high enough to detect.