morning-after pill [AS, morgen + aefter + pilian, to peel,] , (Informal) initially, a reference to a large dose of an estrogen given orally over a short period to a woman within 24 to 72 hours after sexual intercourse to prevent conception, most commonly in an emergency such as rape or incest. The woman is warned that the medication may cause the formation of clots, severe nausea and vomiting, and teratogenic and carcinogenic effects on the fetus if pregnancy already exists or if contraception fails. A newer “morning-after” option, mifepristone, provides an effective method of preventing pregnancy with fewer side effects; a single dose prevents pregnancy by interfering with ovulation or preventing implantation.
