how long does plan b stay effective

Plan B (levonorgestrel emergency contraception) stays effective for preventing pregnancy primarily within 72 hours after unprotected sex, with optimal results when taken as soon as possible. While the drug itself lingers in your system for up to about 5-6 days due to its half-life of around 27.5 hours, its pregnancy-preventing power drops sharply after that initial window.

Effectiveness Timeline

Plan B works best in the first 24 hours (up to 95% effective), remains about 85% effective at 24-48 hours, and around 58% at 49-72 hours.

• 0-24 hours : Highest efficacy, reducing pregnancy risk by 95%.

• 24-72 hours : Still viable but less potent; FDA-approved limit.

• Beyond 72 hours : Not reliably effective—consider alternatives like ella (up to 120 hours) or copper IUD (up to 5 days).

Taking it later than 72 hours doesn't mean zero chance, but studies show diminishing returns as sperm can survive up to 5 days.

How It Stays in Your Body

Your body clears Plan B after roughly five half-lives, totaling under 6 days on average, though this varies by metabolism, weight, and liver function.

• Peak activity aligns with the first half-life (27.5 hours), when hormone levels are strongest.

• By day 5-6, plasma levels are negligible, per pharmacokinetic data.

Factors like BMI over 25-30 can reduce effectiveness, so heavier individuals might need alternatives.

Real-World Considerations

Recent forum chatter (e.g., Reddit's r/birthcontrol) echoes official advice: act fast, but don't rely on Plan B regularly—it's not 100% and disrupts cycles. As of late 2025, no major updates change the 72-hour core window, though access has improved via OTC sales.

Key Tip : Test for pregnancy if your period is over a week late, regardless.

TL;DR : Plan B is most effective within 72 hours (ideally ASAP), detectable up to 6 days, but don't count on it past 3 days.

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