Levonelle - Effectiveness

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.