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how long after sex is plan b effective

Plan B (the levonorgestrel morning‑after pill) is most effective when taken as soon as possible after sex, ideally within 24 hours, and is designed to be used within 72 hours (3 days), but may still offer some protection up to 120 hours (5 days) with reduced effectiveness. It is emergency contraception, not a regular birth control method, and does not work if someone is already pregnant.

Quick Scoop

  • Best timing:
    • Take Plan B as soon as you can after unprotected sex; within 24 hours is when it works at its peak.
* Most guidance treats 72 hours (3 days) as the main effectiveness window.
  • Extended window:
    • Some sources note it may still work up to 120 hours (5 days), but success drops the longer you wait.
* By days 4–5, it’s much less reliable, so earlier is always better.
  • Effectiveness rates (approximate):
    • Within 24 hours: often cited around 95% effectiveness at preventing pregnancy for that act of sex.
* Within 72 hours: overall about 75–89% effective, or roughly 7–8 out of 10 pregnancies prevented.

* After 72 hours up to 120 hours: can still help, but no precise, high numbers are guaranteed and protection is significantly lower.
  • Key limitations:
    • Works mainly by delaying ovulation; if ovulation has already happened and fertilization occurs, it may not prevent pregnancy.
* It will not end an existing pregnancy and is different from abortion pills.

* Weight, timing, and where you are in your cycle can influence how well it works.

What this means in practice

  • If you are within 24 hours after sex:
    • Take Plan B right away; this is your strongest window for protection.
  • If you are 1–3 days (24–72 hours) after sex:
    • Still take it as soon as possible; it is still considered effective emergency contraception in this window.
  • If you are 3–5 days (72–120 hours) after sex:
    • You can still consider taking it, but understand the chance of it working is lower.
* In many places, another emergency contraception option (a prescription pill like ulipristal acetate or a copper IUD) can be more effective this late and even up to the full 5‑day window, so medical advice is important.
  • After 5 days :
    • Plan B is not expected to be effective; you should speak with a healthcare professional about your options and timing for a pregnancy test.

If you’ve already taken it

  • Mild side effects like nausea, fatigue, or changes in the next period are common and usually short‑term.
  • If your period is more than about a week late, or you have pregnancy symptoms, taking a pregnancy test and/or contacting a clinician is recommended.

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