Plan B Timing Chart: Exactly How Efficacy Drops Between Hour 1 and Hour 72
By Robert Thorne
The mathematical probability of pregnancy changes rapidly between the first hour following unprotected sex and the end of the third day. World Health Organization (WHO) collaborative studies and independent clinical trials consistently show a steep degradation curve for levonorgestrel regimens.
During the first 24 hours, levonorgestrel reduces the risk of pregnancy by an estimated 95%. Between 25 and 48 hours, that protection falls to roughly 85%. By hours 49 to 72, efficacy slides to between 58% and 61%. After the 72-hour mark, package labeling states the pill is no longer reliable, though minor marginal benefits have been observed up to 120 hours in emergencies where no other method exists.
| Elapsed Time | Plan B Efficacy (Levonorgestrel) | Ella Efficacy (Ulipristal Acetate) | Copper IUD Insertion |
|---|---|---|---|
| 0, 24 Hours | ~95% risk reduction | ~85%, 90% risk reduction | >99.9% effective |
| 25, 48 Hours | ~85% risk reduction | ~85%, 90% risk reduction | >99.9% effective |
| 49, 72 Hours | ~58%, 61% risk reduction | ~85%, 90% risk reduction | >99.9% effective |
| 73, 120 Hours (Day 4, 5) | Not clinically recommended | ~80%, 85% risk reduction | >99.9% effective |
Every additional hour grants your ovaries time to mount an endocrine response. Delaying a dose to the following morning because the pharmacy is closed or because of minor inconvenience doubles your statistical risk of contraceptive failure. If access is immediate, take it immediately.
Robert Thorne covers electric vehicle innovations, autonomous driving systems, global mobility trends, and automotive engineering developments.
