The morning-after pill may fail under certain circumstances. Remember that is not an abortion pill only prevents pregnancy, if it has not given, by making thicker cervical mucus and prevent contact of sperm with the egg.
Regardless of the brand you choose these are the reasons why the morning-after pill may fail
St John’s wort or St. John’s wort
Do not forget that emergency contraception is not a contraceptive, and should use it only in extraordinary cases. On the other hand, does not protect against diseases / sexually transmitted infections.
Performs Test: Am I pregnant?
Sources; Planned Parenthood Federation of America Inc. Morning-After Pill (Emergency Contraception Retrieved on 7 January 2014; Onmeda: The effectiveness of the morning-after pill Retrieved on January 7, 2014..