Plan B is most effective when taken within 72 hours (3 days) of unprotected sex, but it can still offer some protection up to 120 hours (5 days). The sooner you take it, the better it works. Within that 72-hour window, Plan B prevents pregnancy 81–90% of the time, with effectiveness dropping steadily as hours pass.
How the Time Window Breaks Down
Plan B’s effectiveness isn’t a cliff edge at 72 hours. It’s a sliding scale that starts declining from the moment you have unprotected sex. Taking it within the first 24 hours gives you the best chance of preventing pregnancy. By 48 to 72 hours, it still works well but less reliably. From 73 to 120 hours, it continues to offer some protection, but effectiveness drops further.
The CDC’s 2024 contraceptive guidelines confirm that emergency contraceptive pills can be taken up to 5 days after unprotected sex. However, within that 3-to-5-day window, a different emergency contraceptive pill (sold under the brand name ella) has been shown to work better than Plan B. If you’re past the 72-hour mark, ella is the stronger option, though it requires a prescription.
How Plan B Actually Prevents Pregnancy
Plan B works by delaying or stopping ovulation, the release of an egg from the ovary. Without an egg available, sperm have nothing to fertilize. That’s the entire mechanism. According to the FDA, evidence does not support the idea that Plan B affects fertilization or implantation. It also will not end an existing pregnancy. If you’ve already ovulated and the egg has been fertilized, Plan B is unlikely to help.
This is why timing matters so much. Plan B is essentially racing against your body’s ovulation schedule. If you take it early enough to block the egg’s release, it works. If ovulation has already happened, the window has closed regardless of how many hours it’s been since sex.
How Long Plan B Stays in Your Body
The active ingredient in Plan B reaches its peak concentration in your blood about 2 hours after you swallow the pill. From there, levels drop with a half-life of roughly 26 to 27 hours, meaning about half the drug is eliminated every day. Within a few days, the amount in your system is negligible. Plan B is not a lingering medication, and it doesn’t provide ongoing contraceptive protection. If you have unprotected sex again after taking it, you’re not covered.
Side Effects on Your Next Period
Plan B can shift the timing of your next period by up to a week, either earlier or later than expected. Some people experience light spotting between periods or heavier bleeding than usual when their period does arrive. Nausea is also common in the hours after taking the pill. These effects are temporary and resolve on their own within one cycle. If your period is more than a week late, a pregnancy test is a reasonable next step.
Body Weight Can Reduce Effectiveness
Plan B becomes less effective at higher body weights. Clinical guidelines from the Faculty of Sexual & Reproductive Healthcare note that effectiveness drops noticeably in people who weigh more than about 155 pounds (70 kg) or have a BMI above 26. If that applies to you, a copper IUD inserted within 5 days is the most effective emergency contraception option at any weight. The prescription pill ella is another alternative that holds up better across a wider weight range.
Medications That Interfere With Plan B
Certain medications speed up how quickly your body breaks down Plan B, potentially making it less effective. These include several anti-seizure medications (such as carbamazepine, phenytoin, and topiramate), the antibiotic rifampin, and the herbal supplement St. John’s wort. If you take any of these regularly and need emergency contraception, a copper IUD is a more reliable choice since its effectiveness doesn’t depend on hormone levels in your blood.
Plan B vs. Other Emergency Options
Plan B is available over the counter without a prescription or age restriction, which makes it the most accessible emergency contraceptive. But it’s not always the best one. Here’s how the main options compare:
- Plan B (levonorgestrel): Over the counter. Most effective within 72 hours. Reduced effectiveness above 155 lbs.
- ella (ulipristal acetate): Prescription only. Maintains stronger effectiveness through the full 5-day window, especially on days 3 through 5. Better option at higher body weights than Plan B.
- Copper IUD: Requires a clinic visit for insertion. Over 99% effective when placed within 5 days. Works at any body weight and provides ongoing contraception for up to 10 years.
The bottom line: Plan B works best in the first 72 hours and loses ground after that. If you’re within that window and don’t have weight-related concerns, it’s a solid option. Outside that window, or if other factors reduce its reliability, the alternatives are worth considering.