Plan B is most effective when taken as soon as possible after unprotected sex, and it is generally considered effective only within a limited time window, not long term.
Time window for effectiveness
- Plan B (levonorgestrel) is approved to be used within 72 hours (3 days) after unprotected sex; its ability to prevent pregnancy drops the longer you wait in that 72‑hour period.
- Some data suggest there may still be some reduced effectiveness up to 120 hours (5 days), but it is significantly less reliable after 72 hours, and other emergency options (like a copper IUD or a different pill) are usually recommended after that point.
How long it works in your body
- The hormone in Plan B stays in your system for about 5–6 days, but it does not keep protecting you from new unprotected sex during that whole time; it mainly works by delaying ovulation around the time you take it.
- Plan B will not work if you were already ovulating or already pregnant, and it does not protect against pregnancy from sex that happens after you take it.
What to do if you’re past the window
- If it has been more than 72 hours (and especially more than 5 days) since unprotected sex, Plan B is unlikely to be effective, and you should talk to a clinician or clinic as soon as you can about other emergency contraception options (such as a copper IUD or ulipristal acetate, if available where you live).
- If you already took Plan B, take a pregnancy test if your next period is more than about a week late or at least 3 weeks after the unprotected sex, and seek medical care if you have severe pain or unusual symptoms.
