Princeton University maintains a website with answers to many frequently asked questions about emergency contraception (EC).  Follow the link below to learn more about the types of birth control pills that can be used for EC in the United States:

http://ec.princeton.edu/questions/dose.html

NOTE: The above link is operated by the Office of Population Research at Princeton University and by the Association of Reproductive Health Professionals and has no connection with any pharmaceutical company or for-profit organization. This website is peer reviewed by a panel of independent experts.  However, NARAL Pro-Choice Texas bears no responsibility for the accuracy or results of following the advice of the information contained therein