Dubbed the "French abortion pill" and the "month after pill", the pharmaceutical RU-486 was released to wide acceptance in Europe in 1988, but was delayed for licensing in the United States by the Food and Drug Administration until September 2000, because of political pressure against abortions. In France, where the drug was developed, about one-third of all abortions are performed using the synthetic steroidal hormone.

Known to doctors as mifepristone and to pharmacists by the trade name Mifeprex, RU-486 is a synthesized prostaglandin that interferes with progesterone metabolism. By making the uterine lining inhospitable to the developing embryo, the zygote fails to attach, is deprived of nutrients, and is miscarried. Administered as a three-tablet regimen, a mifepristone dose is taken first, followed two days later by two misoprostol tablets, the latter causing contractions and expulsion of the uterus' contents. As an at-home option and alternative to surgery, RU-486 provides a private procedure for terminating an early pregnancy. It can also function as a "morning-after pill" and as a menstrual initiator.

Originally named RU 38486 by French pharmaceutical company Roussel-Uclaf (thus "RU"), mifepristone forms a yellow powder with a molecular weight of 429.6, a melting point of 191 to 196 degrees Celsius, and is insoluble in water or hexane and very soluble in methanol, chloroform, and acetone. RU-486 acts as an antagonist for progesterone by binding to the progesterone site, tying itself to the hormone, and blocking its release. Mifepristone is a substituted 19-nor steroid compound designated by organic chemists as 11beta-[p-(Dimethylamino)phenyl]-17beta-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one.

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