The manufacturer determines the effectiveness of the level about 84% of expected pregnancies when it is taken within 72 hours (3 days) of unprotected sex. A trial undertaken by the World Health Organization (WHO) indicated that levonorgestrel (the drug in Levonelle) prevented:
95% of expected pregnancies when taken within 24 hours of sex
85% if taken within 25-48 hours
58% if taken within 49-72 hours
More recent studies suggest that the prevention rate might be lower, but still substantial.