Plan B (levonorgestrel emergency contraception) begins working soon after you take it, reaching peak levels in your bloodstream about 1.7 hours after ingestion
. It works primarily by delaying ovulation for a few days, which helps prevent pregnancy if taken before ovulation occurs
. For best effectiveness, Plan B should be taken as soon as possible after unprotected sex-ideally within 12 to 24 hours, when it can be about 95% effective
. It remains effective if taken within 72 hours (3 days), reducing the chance of pregnancy by 75–89%, but its efficacy decreases the longer you wait
. It can be taken up to 5 days after sex, but with much lower effectiveness
. In summary:
- Plan B starts working within about 1.7 hours after taking it.
- It delays ovulation for a few days to prevent pregnancy.
- Take it as soon as possible, ideally within 24 hours for maximum effectiveness.
- It is FDA-approved for use up to 72 hours after unprotected sex.
- Effectiveness decreases the longer you wait, with some benefit up to 5 days but much less reliable beyond 3 days.
If you do not get your period within three weeks after taking Plan B, you should take a pregnancy test
