Levonorgestrel in cases of rape: How does it work?

The Ethical and Religious Directives for Catholic Health Care Services allows the use of an emergency contraceptive for a woman who has been raped, as a defense against her attacker's sperm, provided the drug prevents fertilization and does not act against a conceived human life. Catholic emergency rooms around the country have been pressured to provide Plan B (LNG-EC) to patients seeking help after a sexual assault. Catholic bioethicists have supported the use of this drug based on their interpretation of the scientific literature regarding its mechanism of action.