There are 2 pills you must take 12 hours apart. The pill can either delay ovulation, hinder fertilization, or alter the lining of the uterus, not allowing the implantation of the embryo/fertilized egg. The morning-after pill carries the side effects of regular birth control pills, including: changes in menstrual cycle; nausea; vomiting; headaches; abdominal pain or cramping; weight gain; and hot flashes. Additional side effects of the morning-after pill include: swelling of the face, ankles or feet; unusual tiredness or weakness; acne; trouble sleeping; loss of sexual desire; skin rash; mental depression; blood clot formation; numbness; and ectopic pregnancy.
http://www.cwfa.org/articles/4265/CWA/life/ (an anti-OTC article)