You should take Plan B as soon as possible after unprotected sex. The pill can be taken up to 72 hours (3 days) afterward, but its effectiveness drops significantly with each passing day. A different emergency contraceptive pill is available that works up to 120 hours (5 days) after sex, making it the better option if you’re past the 72-hour mark.
Why Sooner Is Better
Plan B works primarily by delaying or preventing ovulation, the release of an egg from the ovary. If your body hasn’t released an egg yet, the pill can pause that process long enough for sperm (which survive about five days) to die off before they can fertilize anything. The critical detail: if ovulation has already happened, Plan B has little to no effect. Research tracking women with ultrasound and hormone levels found that when the pill was given on the day of ovulation or after, the number of pregnancies matched what would have been expected without any intervention at all.
This is why timing matters so much. Every hour you wait brings you closer to the moment your body may release an egg, and once that happens, the pill essentially can’t help. Taking it within the first 12 to 24 hours gives you the best chance of getting ahead of ovulation.
What Happens After 72 Hours
Plan B’s labeling sets a 72-hour window, but if you’re past that point, you have another option. A prescription emergency contraceptive called ella (ulipristal acetate) remains effective for up to 120 hours, or 5 full days, after unprotected sex. It is more effective than Plan B overall, and the gap between the two widens as time passes. Between 72 and 120 hours, ella substantially outperforms Plan B, though both become less reliable the longer you wait.
Ella requires a prescription in most cases, though some telehealth services can provide one quickly. If you’re within the 72-hour window and can get Plan B immediately, that speed advantage may matter more than the theoretical edge ella provides. If you’re past 72 hours, ella is clearly the better choice.
Body Weight Affects How Well It Works
Plan B becomes less effective at higher body weights. A pooled analysis of four large studies found that women with a BMI of 30 or above had a pregnancy rate of 5.8% after taking the pill, compared to 1.3% for women with a BMI under 25. Women in the overweight range (BMI 25 to 30) fell in between at 2.5%. In practical terms, a woman with a BMI over 30 was roughly four to six times more likely to become pregnant after taking Plan B than a woman at a lower weight.
If your weight is a concern, ella may be a more reliable option regardless of timing. A copper IUD, inserted by a healthcare provider within five days, is the most effective form of emergency contraception at any weight and has the added benefit of providing ongoing birth control for up to 10 years.
What to Expect After Taking It
Plan B is a single pill. You swallow it with or without food, and that’s it. One important caveat: if you vomit within 2 hours of taking the pill, it may not have been fully absorbed and you may need to take another dose.
Side effects are generally mild. Nausea is the most common. Your next period may arrive up to a week late, which can be anxiety-inducing when you’re already worried about pregnancy. Some women also experience spotting, breast tenderness, or fatigue. These typically resolve on their own within a few days.
If your period is more than a week late, take a pregnancy test. A late period alone doesn’t mean the pill failed, but it’s worth confirming.
If You’re Breastfeeding
Plan B is considered safe during breastfeeding. Only small amounts of the medication pass into breast milk. If you want to minimize your baby’s exposure, wait 3 to 4 hours after taking the pill before nursing. You don’t need to pump and dump.
What Plan B Does Not Do
Plan B does not end an existing pregnancy. It works by preventing ovulation or fertilization. Once a fertilized egg has implanted in the uterus, the pill has no effect. It is also not a substitute for regular contraception. Taking it repeatedly is safe, but it’s less effective than consistent use of daily birth control, an IUD, or other long-acting methods. If you find yourself reaching for Plan B frequently, that’s a practical signal to explore a more reliable ongoing option.