Plan B’s active ingredient stays in your system for about a week, but its ability to prevent pregnancy is limited to a narrow window around the time you take it. The pill works by delaying ovulation for roughly five days, and it does not provide ongoing protection against future unprotected sex. Most side effects clear within 24 hours, though some cycle changes can linger for weeks.
How Long Plan B Stays in Your Body
After swallowing the 1.5 mg tablet, levonorgestrel (the active hormone) hits peak levels in your blood within about two hours. From there, it drops steadily with a half-life of roughly 26 to 27 hours, meaning half the drug is gone in just over a day. Within one week of taking a single dose, the medication is fully cleared from your body.
But “in your system” and “actively working” are two different things. The pill’s pregnancy-preventing effect comes from delaying or blocking ovulation, and that delay lasts approximately five days. If you ovulate after the drug has done its job, you’re no longer protected.
Effectiveness Drops Quickly With Time
Plan B is most effective the sooner you take it after unprotected sex. The numbers tell a clear story:
- Within 24 hours: around 94% effective at preventing pregnancy
- Within 72 hours (3 days): about 58% effective
After 72 hours, levonorgestrel-based emergency contraception is not recommended. If you’re past that window, a different option called ulipristal acetate works up to 120 hours (five days) after sex and remains about 85% effective at that outer limit.
It Does Not Cover Future Unprotected Sex
A common misconception is that Plan B offers a buffer of protection for several days after you take it. It does not. The pill addresses one event: the unprotected sex that already happened. If you have unprotected sex again a day or two later, you are at risk of pregnancy, even if Plan B is still technically circulating in your blood. The remaining drug levels are not high enough to reliably suppress a new ovulation cycle triggered by a separate act of intercourse.
If you need ongoing protection, starting a regular contraceptive method immediately after taking Plan B is safe and recommended.
How Long Side Effects Last
Most side effects are mild and resolve within 24 hours. Nausea and cramping are the most common complaints, and for the majority of people they feel no different by the next day. If symptoms persist beyond 48 hours or feel severe, that’s worth a call to a healthcare provider.
The exception is menstrual changes, which can show up days or even weeks later. You might notice spotting before your next period, or your period may arrive earlier or later than expected. The Mayo Clinic notes that Plan B can delay your period by up to one week. If your period is more than a week late, a pregnancy test is a reasonable next step.
Body Weight Can Affect How Well It Works
Plan B’s effectiveness decreases at higher body weights. According to Planned Parenthood, levonorgestrel-based emergency contraception does not work as well for people who weigh more than 165 pounds. It’s not that the pill fails entirely, but the likelihood of it successfully preventing pregnancy drops enough to matter.
If you’re above that threshold, ulipristal acetate or a copper IUD inserted within five days are more reliable alternatives. A copper IUD is the single most effective form of emergency contraception regardless of weight, and it doubles as long-term birth control once placed.
Timeline Summary
- Peak blood levels: about 2 hours after taking the pill
- Ovulation suppression: up to approximately 5 days
- Most side effects gone: within 24 hours
- Drug fully cleared: within 1 week
- Possible period delay: up to 1 week beyond your expected date
Plan B is a single-event safety net, not an extended shield. Its protective effect is concentrated in the hours and days immediately after you take it, with the strongest benefit in the first 24 hours. Once the drug clears and your cycle resumes its normal pattern, its job is done.