Plan B (levonorgestrel) is an emergency contraceptive pill that is most effective when taken as soon as possible after unprotected sex. Its effectiveness varies depending on how quickly it is taken:

• Within 24 hours after unprotected sex, Plan B can be up to 95-98% effective at preventing pregnancy

• Within 72 hours (3 days), it is estimated to be about 60% to 94% effective, with many sources citing around 75% to 89% effectiveness

• Effectiveness decreases the longer you wait, but it may still have some moderate effect if taken up to 5 days (120 hours) after unprotected sex, though this is less certain and not the primary recommendation

Plan B works mainly by delaying ovulation; it is less effective if ovulation has already occurred

. It is not intended as a regular birth control method but as a backup after contraceptive failure or unprotected sex

. Some factors can reduce its effectiveness, such as certain medications and higher body weight or BMI, though it is still recommended to take Plan B if it is the only option available

. In summary, Plan B is quite effective-up to about 89-95%-when taken promptly within 24 hours, and still reasonably effective within 72 hours, but its efficacy declines over time and it is not as reliable as regular contraception