Plan B is most effective within the first 24 hours after unprotected sex, when it prevents pregnancy about 94% of the time. Its effectiveness drops steadily from there: around 85% at 48 hours and roughly 58% by 72 hours. The labeled window is three days (72 hours), though there is some reduced protection out to five days.
Effectiveness by Time Window
The single most important factor in how well Plan B works is how quickly you take it. Every hour matters, but the decline in effectiveness is most dramatic across the first three days:
- Within 24 hours: approximately 94% effective
- Within 48 hours: approximately 85% effective
- Within 72 hours: approximately 58% effective
- 73 to 120 hours: still provides some protection, but significantly less
That steep drop from 94% to 58% over just three days is why pharmacists and doctors stress taking it as soon as possible. If you’re past 72 hours, Plan B can still be taken, but a different emergency contraceptive may be a better option at that point (more on that below).
How Plan B Actually Works
Plan B contains a single 1.5 mg dose of levonorgestrel, a synthetic hormone. It prevents pregnancy by delaying or blocking ovulation, the release of an egg from the ovary. Without an egg available, sperm have nothing to fertilize.
This is why timing matters so much. Plan B needs to act before your body releases an egg. A large review published in the journal Contraception found no evidence that Plan B affects implantation or works after ovulation has already occurred. It delays ovulation by at least five days, which is long enough for sperm (which survive roughly five days in the reproductive tract) to die off before an egg appears.
After you take it, the drug reaches peak levels in your blood within about two hours. It then clears with a half-life of roughly 26 to 27 hours, meaning it takes a little over a day for half the dose to leave your system. Within a few days, it’s largely gone.
Why It Doesn’t Work During Ovulation
If you’ve already ovulated, Plan B is unlikely to prevent pregnancy. The drug works by stopping the hormonal surge that triggers egg release, but once that process is underway, it can’t reverse it. This is the biggest gap in Plan B’s protection and the main reason it fails in some cases.
The challenge is that most people don’t know exactly when they’re ovulating. Ovulation typically happens around the midpoint of a menstrual cycle, but it can shift by several days depending on stress, sleep, illness, or cycle irregularity. If you suspect you may have already ovulated, a prescription alternative called ella (ulipristal acetate) is significantly more effective. In clinical trials, ella was able to prevent egg release 79% of the time even when taken during the hormonal surge that immediately precedes ovulation. Plan B managed only 14% in the same scenario.
Body Weight Can Reduce Effectiveness
Research has shown that Plan B becomes less reliable in people with a BMI above 26. That threshold is lower than many people expect. For reference, a BMI of 26 corresponds to about 155 pounds for someone who is 5’4″, or about 170 pounds at 5’7″.
The exact reason isn’t fully understood, but higher body weight appears to dilute the drug’s concentration in the bloodstream enough to reduce its ability to block ovulation. If your weight falls in this range, a copper IUD (which can be placed up to five days after unprotected sex) is considered the most effective emergency option regardless of body size. Ella also maintains better effectiveness at higher body weights than Plan B does.
How Plan B Compares to Ella
Within the first 72 hours, Plan B and ella have similar overall effectiveness. The real difference shows up in two situations: when more than three days have passed, and when you’re close to ovulation.
Ella is labeled as effective for up to five full days (120 hours) after unprotected sex, and pooled data from three randomized trials found that women who took ella were significantly less likely to become pregnant than those who took Plan B. Ella requires a prescription, while Plan B is available over the counter. If you’re within 24 hours and confident you haven’t ovulated, Plan B is a solid choice. If you’re past 48 hours, closer to ovulation, or at a higher body weight, ella or a copper IUD offers a meaningful advantage.
Medications That Interfere With Plan B
Certain drugs speed up how your liver processes levonorgestrel, which can reduce Plan B’s effectiveness or shorten the window in which it works. The most notable ones include the antibiotic rifampin, the antifungal griseofulvin, certain HIV medications, some anti-seizure drugs (which are also prescribed for conditions like bipolar disorder), and the herbal supplement St. John’s Wort. Other common antibiotics and antifungals do not interfere with Plan B.
If you take any of these regularly, a copper IUD is the most reliable emergency contraceptive because it works locally in the uterus rather than depending on hormone levels in the bloodstream.
What Plan B Doesn’t Do
Plan B is a single-use emergency measure, not ongoing protection. It does not prevent pregnancy from sex that happens after you take it. If you have unprotected sex again even a day later, you’d need another dose or a different contraceptive method. It also does not protect against sexually transmitted infections and does not terminate an existing pregnancy. Its only job is to delay ovulation long enough to prevent fertilization from a single act of unprotected sex that already happened.