Plan B (levonorgestrel emergency contraception) only protects you from pregnancy for the unprotected sex that happened before you took it, and its effect is short-term, over a few days, not weeks or months. It does not keep working to protect you from any sex you have after taking it.

How long Plan B is effective

  • Plan B is approved to be taken within 72 hours (3 days) after unprotected sex, and it is most effective the sooner you take it (ideally within 24 hours).
  • Some evidence suggests levonorgestrel emergency pills can still reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but effectiveness drops the longer you wait.
  • Once you swallow the pill, the hormone is largely out of your system within a few days, so its protective window is basically just for that one incident, during the fertile window when sperm could meet an egg.

What it does and doesn’t do

  • Plan B mainly works by delaying or preventing ovulation so that sperm (which can live up to 5 days inside the body) cannot meet an egg.
  • It will not end an existing pregnancy and will not protect you from pregnancy from sex you have after you take it.
  • It also does not protect against STIs, so condoms or other barrier methods are still important.

When to be concerned

  • If your period is more than about 1 week later than expected (or has not come within 3 weeks of taking Plan B), taking a pregnancy test is recommended.
  • If you vomit within 2 hours of taking Plan B, the dose may not be fully absorbed, and you may need another dose after talking to a clinician or pharmacist.

If it has been more than 3–5 days

  • If unprotected sex was more than 3 days ago (or especially more than 5 days), Plan B is much less likely or unlikely to help for that encounter.
  • In that case, copper IUD emergency contraception or a different prescription emergency pill (ulipristal, often called Ella) may be more effective options if still within 5 days, so speaking with a health professional or clinic urgently is recommended.

Important: If you are worried about pregnancy right now, note the date and time of unprotected sex, when you took Plan B (if you did), and contact a clinician, urgent care, or a sexual health clinic as soon as possible for personalized advice.