Plan B (the levonorgestrel “morning‑after pill”) is most effective when taken as soon as possible after unprotected sex, and its ability to prevent pregnancy drops the longer you wait within the next few days.

Core timing: how long it’s effective

  • Plan B is designed to be taken within 72 hours (3 days) after unprotected sex; this is the officially recommended window where it works best.
  • Within that 72‑hour window, effectiveness is highest in the first 24 hours and then gradually declines each day.
  • Some clinical sources note that levonorgestrel emergency contraception can still help reduce pregnancy risk up to 120 hours (5 days), but protection is clearly weaker after 72 hours, so another method (like a copper IUD or the pill ulipristal/ella) is usually preferred beyond 3 days.

What “effective” really means

  • Plan B does not provide ongoing birth control; it only works for the single episode of unprotected sex that already happened by delaying ovulation so sperm and egg do not meet.
  • Once ovulation has already occurred, Plan B is much less likely to work, which is why timing within the cycle matters even if you took it quickly.
  • It does not protect you from unprotected sex that happens later in the same cycle; you would need regular contraception (like pills, IUD, implant, condoms) for ongoing protection.

How long it stays in your body

  • Levonorgestrel from Plan B reaches peak levels in the blood in about 2 hours and is mostly cleared from your system over about 5–7 days.
  • The “active” pregnancy‑prevention effect is essentially limited to the first few days after you take it; after that, your hormones and cycle gradually return to baseline.
  • Because of this, taking multiple doses for the same encounter will not make it more effective and is not recommended.

If you’re past 72 hours

  • Between 72 and 120 hours (3–5 days), levonorgestrel pills like Plan B may still reduce the chance of pregnancy somewhat, but a copper IUD or ulipristal acetate (ella) are typically more effective options within that time frame.
  • If you are already beyond 5 days from unprotected sex, emergency contraception pills are very unlikely to help and a pregnancy test plus a consultation with a clinician or clinic (like Planned Parenthood) is the next step.
  • Any time your period is more than about a week late after taking Plan B, taking a home pregnancy test and following up with a professional is recommended.

Quick TL;DR

  • Best effectiveness: within 24 hours after unprotected sex.
  • Recommended window: up to 72 hours (3 days), with decreasing effectiveness each day.
  • Possible but less reliable help: up to 120 hours (5 days), and other methods are usually better after 3 days.

Information gathered from public forums or data available on the internet and portrayed here.