It depends where a woman is in her cycle. If she has ovulated, then most likely it won’t work.
It also depends WHICH emergency contraceptive pill. If you take the over the counter one which is levonorgestrel 1.5 mg vs the by prescription only ulipristal acetate – the latter is MORE effective on days 3-5 after an accident and more effective in women who are 177 lbs or more.
Regardless of efficacy, if you have a contraceptive failure or are sexually-assaulted and want to avoid pregnancy and are within 5 days of the event, it doesn’t hurt to try to prevent pregnancy by using Emergency Contraception.
All Emergency Contraception(EC) is not 100% effective.
Levonorgestrel EC has a variable efficacy from 75–95% effective depending where you are in your cycle and also if you weigh 75 -79 kg, Levonorgestrel EC has lower efficacy than if you weigh <75kg. If you weigh 80kg or more, we recommend you use ulipristal acetate or copper IUD and NOT the Levonorgestrel EC.