Plan B prevents pregnancy about 85% to 89% of the time when taken within 72 hours of unprotected sex. That number represents the proportion of expected pregnancies it prevents, not the chance you’ll get pregnant after taking it. In clinical studies, only 1.2% to 2.1% of women who took levonorgestrel emergency contraception actually became pregnant. The sooner you take it, the better it works.
How Timing Affects the Success Rate
Plan B is most effective when taken as soon as possible after unprotected intercourse. Emergency contraception can prevent over 95% of expected pregnancies when taken within the first day or two. That effectiveness drops steadily with each passing day. By 72 hours, it still works but noticeably less well. The pill can technically be taken up to 120 hours (5 days) after sex, though by that point its reliability has decreased significantly.
The reason timing matters so much is that Plan B works primarily by delaying or preventing ovulation. If your body hasn’t released an egg yet, the pill can pause that process long enough for sperm to die off. But if ovulation has already happened, Plan B has little to no ability to prevent a pregnancy. This is the single biggest factor in whether the pill succeeds or fails: where you are in your cycle when you take it.
Why Body Weight Changes the Equation
Plan B becomes less effective at higher body weights. According to Planned Parenthood, if you weigh more than 165 pounds, levonorgestrel-based emergency contraception won’t work as well. This doesn’t mean it’s completely ineffective, but the protection drops enough that other options are worth considering.
For people in this weight range, two alternatives offer better protection. Ulipristal acetate (sold as ella) is more effective than Plan B at all time points within 5 days and maintains its effectiveness at higher weights, though it also begins losing efficacy above 195 pounds. A copper IUD is the most effective form of emergency contraception regardless of body weight, and it can be inserted up to 5 days after unprotected sex. It also doubles as ongoing birth control for up to 10 years.
How Plan B Compares to Other Emergency Options
The American College of Obstetricians and Gynecologists ranks emergency contraception options in this order of effectiveness:
- Copper IUD: the most effective option for all women, regardless of weight or timing within the 5-day window.
- Ulipristal acetate (ella): more effective than Plan B at every time point up to 120 hours. Requires a prescription.
- Levonorgestrel (Plan B): available over the counter without ID or age restrictions. Less effective than the other two options but the most accessible.
Plan B’s main advantage is convenience. You can buy it at most pharmacies without a prescription. That accessibility makes it the most commonly used emergency contraceptive, even though it isn’t the most effective one available.
What Can Make Plan B Less Effective
Beyond timing and weight, certain medications can reduce how well Plan B works. These include some anti-seizure medications, certain antibiotics, anti-HIV drugs, and the herbal supplement St. John’s wort. These substances speed up how quickly your liver breaks down the hormone in Plan B, reducing the amount that stays active in your body. If you take any of these regularly, a copper IUD is a more reliable emergency option.
Vomiting shortly after taking the pill is another practical concern. If you throw up within 2 hours of swallowing Plan B, your body may not have absorbed enough of the medication. In that case, you likely need to take another dose. A pharmacist can help you figure out whether a second pill is necessary.
What to Expect After Taking It
Common side effects include nausea, headache, and cramping similar to period pain. These typically resolve within a day or two. Plan B can also shift the timing of your next period by up to a week in either direction. An early or late period after taking emergency contraception is normal and not a sign that something went wrong.
The tricky part is that a late period can also be an early sign of pregnancy, which creates understandable anxiety. The reliable way to know whether Plan B worked is to take a pregnancy test three weeks after you took the pill. Testing earlier than that can produce inaccurate results because pregnancy hormones need time to build to detectable levels. If your period is more than a week late and you haven’t tested yet, that’s a good time to pick one up.
Putting the Numbers in Context
The 85% to 89% effectiveness figure can sound lower than people expect, but it’s important to understand what that number actually measures. It refers to the percentage of expected pregnancies prevented, not the percentage of women who get pregnant. In any given cycle, the baseline chance of pregnancy from a single act of unprotected sex is roughly 5% to 8%, depending on where you are in your cycle. Plan B reduces that already-small probability by the majority. The actual pregnancy rate in clinical studies, 1.2% to 2.1%, reflects how many women became pregnant after taking it.
If you took Plan B within the first 24 hours, you’re on the higher end of its protection. If you took it closer to 72 hours and weigh under 165 pounds, you’re still well-protected but with somewhat lower odds. And if timing, weight, or medications put you in a less ideal category, knowing that options like ella and the copper IUD exist gives you a backup plan for next time.