Answer:
The levonorgestrel morning after pill, such as Plan B or Postinor, should be taken within 120 hours of unprotected vaginal intercourse. It is is most effective when it is taken as soon as possible after unprotected sex. Taken within 24 hours of the accident, they reduce the risk of pregnancy by 95%, and within 72 hours, by 88%. That means that even on your most fertile day, you can lower the risk of pregnancy to 0.5% to 1.5% by taking the morning after pill in the first 24 hours. There are no reliable data on the effectiveness between 72 and 120 hours.
A new medication, Ella, can be taken up to five days after sex without any decrease in effectiveness.