Plan B is most effective when taken as soon as possible after unprotected sex, and taking it within 12 hours puts you at the high end of its effectiveness range. Overall, Plan B is estimated to be 60% to 94% effective at preventing pregnancy when taken within 72 hours, but effectiveness is highest in the first 24 hours and declines steadily from there. Within the first 12 hours, you’re getting the best performance the pill can offer.
Why Sooner Means More Effective
Plan B works primarily by delivering a large dose of a synthetic hormone that delays ovulation. If your body hasn’t yet released an egg, the drug can suppress the hormonal surge that triggers that release, preventing fertilization entirely. The key word is “delay”: once ovulation is already underway, the drug has a much harder time stopping it. In one study, women who took the medication before their hormonal surge still ovulated normally in more than half of cases, likely because ovulation was already too far along to interrupt.
This is why the clock matters so much. Every hour that passes is an hour closer to potential ovulation. Taking Plan B within 12 hours gives the drug the widest possible window to get ahead of that process. By 72 hours, effectiveness drops considerably, with failure rates climbing from under 1% at the early end to roughly 2.6% at the outer limit.
What the Numbers Actually Mean
The 60% to 94% effectiveness range can be confusing because it sounds imprecise. That range reflects the reality that not every act of unprotected sex carries the same pregnancy risk. If you’re far from ovulation, your baseline risk is already low, and Plan B adds relatively little protection. If you’re close to your fertile window, the drug is doing much heavier lifting. The 94% figure represents the best-case scenario: taking the pill quickly when it has the most opportunity to prevent ovulation.
In practical terms, somewhere between 0.6% and 2.6% of women who take Plan B within 72 hours still become pregnant. When you take it within 12 hours, your odds sit closer to that 0.6% end. That’s not a guarantee, but it’s a significant reduction in risk.
Body Weight Can Reduce Effectiveness
One factor many people don’t know about: Plan B becomes less effective at higher body weights. Clinical guidelines from the Faculty of Sexual and Reproductive Healthcare indicate that the standard dose appears to be less effective in women with a BMI above 26 or a body weight above about 155 pounds (70 kg). If that applies to you, a different emergency contraceptive called ella (ulipristal acetate) is a stronger option regardless of timing. Ella is more effective than Plan B across the board and maintains its effectiveness better over the full five-day window. It does require a prescription in most cases.
For women above that weight threshold who can’t access ella, some guidelines suggest a double dose of levonorgestrel (the active ingredient in Plan B), though the effectiveness of that approach hasn’t been well studied.
Medications That Interfere With Plan B
Certain medications speed up how quickly your liver breaks down Plan B’s active ingredient, which can reduce blood levels enough to undermine its effectiveness. The main culprits include medications used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampicin, some HIV medications, and the antifungal griseofulvin. The HIV drug efavirenz, for example, reduces blood levels of the active ingredient by about 50%.
St. John’s wort, a popular herbal supplement, has the same effect. And this isn’t just about what you’re currently taking. The enzyme changes these drugs cause can persist for up to four weeks after you stop taking them. If any of these apply to you, a copper IUD inserted as emergency contraception is the most reliable alternative.
What to Expect After Taking It
Side effects are generally mild. You may experience nausea, fatigue, headaches, breast tenderness, or cramping in the hours and days after taking the pill. Your next period may arrive earlier or later than expected, and you might notice some light spotting before it does. These are all normal responses to a large dose of synthetic hormone and typically resolve on their own.
If you vomit within three hours of taking the pill, the dose may not have been fully absorbed. In that case, you likely need to take another dose. Taking Plan B with food or a small snack can help reduce nausea.
Plan B vs. Ella
If you’re reading this and haven’t taken anything yet, it’s worth knowing that ella outperforms Plan B at every time point, including the first 12 hours. Ella works by a different mechanism that is more effective at blocking ovulation even when the hormonal surge has already started. It also maintains its effectiveness over a longer window (up to five days) without the steep drop-off that Plan B shows after 24 to 48 hours. The tradeoff is access: ella requires a prescription, while Plan B is available over the counter at most pharmacies with no age restriction.
If you already have Plan B on hand and you’re within 12 hours, take it now. The difference in effectiveness between Plan B taken immediately and ella taken hours later while you wait for a prescription can easily tip in Plan B’s favor. Speed is the single most important variable in emergency contraception.