Plan B (levonorgestrel emergency contraception) is most effective the sooner you take it, and its effectiveness drops with each passing hour after unprotected sex. It is not meant to be used if more than about 3–5 days have already passed since the risk of pregnancy.

Core timing answer

  • Plan B is FDA‑approved to be taken within 72 hours (3 days) after unprotected sex or birth control failure.
  • It can still have some effect up to 120 hours (5 days), but protection is significantly lower as you get closer to day 5.
  • After about 5 days from the unprotected sex, Plan B is considered no longer effective for that specific episode and will not prevent pregnancy that may already have occurred.

How effectiveness changes over time

  • Within 24 hours: highest protection, with some sources noting up to about 95% effectiveness when taken very quickly after sex.
  • Within 72 hours: still reasonably effective (around 75–89% risk reduction), but clearly less than in the first 24 hours.
  • Between 72 and 120 hours: may still help, but success rates are lower and less predictable, and other options like a copper IUD can be more effective if available.

Important limits and caveats

  • Plan B only works before ovulation; if ovulation and fertilization have already happened, it will not stop an existing pregnancy.
  • It is not an abortion pill and will not end a pregnancy that has already implanted.
  • Some medicines (like certain seizure drugs, rifampin, St. John’s wort) and higher body weight can make Plan B less effective, so timing is not the only factor.

What to do if it’s “too late”

  • If it has been more than 5 days since the unprotected sex, Plan B is unlikely to help for that encounter; a pregnancy test is recommended if your period is more than about a week late.
  • If you are still within 5 days, but Plan B timing worries you, a copper IUD placed by a clinician within 5 days of unprotected sex is one of the most effective emergency options and then works as ongoing contraception.

If you are very anxious, have irregular periods, or had unprotected sex multiple times in a cycle, contacting a healthcare provider or clinic as soon as possible is strongly recommended for individualized advice.

TL;DR: Plan B is designed for use within 3 days, may help up to 5 days, and after that point it is generally considered not effective for that specific episode of unprotected sex.

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