Plan B (levonorgestrel emergency contraception) works in your body for about 5 days after you take it , which matches the time sperm can survive in the reproductive tract. Its effective window for preventing pregnancy is strongest in the first 72 hours after unprotected sex and then decreases, but some protection can extend up to 5 days after sex.

Quick Scoop

  • Plan B is designed as a one‑time, short‑term backup, not ongoing birth control.
  • It works best when taken as soon as possible , ideally within 24 hours of unprotected sex.
  • The hormone from Plan B is generally active in your body for up to about 5 days , roughly the same time sperm can live inside the uterus and fallopian tubes.

How long Plan B works after taking it

  • Plan B is most effective if taken within 72 hours (3 days) of unprotected sex, reducing the risk of pregnancy by about 75–89%.
  • You can take it up to 5 days after sex , but effectiveness drops the later you take it, especially after day 3.
  • Once you swallow the pill, its hormone effect lasts for around 5 days , after which it’s essentially out of your system and no longer preventing pregnancy.

Think of it this way: Plan B is a 5‑day shield that works best when you raise it immediately after unprotected sex, then slowly weakens over those days.

What it does (and doesn’t) do

  • Plan B mainly works by delaying or blocking ovulation , so sperm never meets an egg.
  • If ovulation has already happened and an egg has been fertilized and implanted, Plan B will not end a pregnancy ; it is not an abortion pill.
  • It does not protect you from any new unprotected sex after you take it; each new risk would need its own emergency contraception or regular birth control.

After you take Plan B: what to watch

  • Your next period may come a few days earlier or later , and flow may be lighter or heavier than usual.
  • Common short‑term side effects include nausea, fatigue, headache, breast tenderness, and light spotting , usually resolving within a few days.
  • If your period is more than about 1 week late after taking Plan B, or you do not get a bleed within about 3 weeks, taking a pregnancy test is recommended.

Key timing questions people ask

  • “Does Plan B keep working for the rest of my cycle?”
    • No. Its main effect is limited to those several days after you take it; it does not keep you protected for the whole month.
  • “If I have sex again a few days after Plan B, am I still covered?”
    • No. Plan B does not provide ongoing protection for later sex; you would need another dose (if appropriate) or a reliable regular method like the pill, implant, IUD, or condoms.

Bottom line / TL;DR:
Plan B works best when taken ASAP , is maximally effective within 3 days , and its protective action lasts for roughly 5 days after you take it , after which it no longer offers pregnancy prevention.

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