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mifepristone mi·fep·ri·stone (mĭ-fěp'rĭ-stōn)
synthetic steroid drug used under various trade names (e.g., RU-486, Mifegyne, Mifeprex) to induce abortion in the early weeks of pregnancy. Mifepristone is an antiprogestin; that is, it blocks the action of progesterone, a naturally produced hormone that prepares the inner lining of the uterus for implantation of a fertilized ovum and support of a growing embryo and placenta. The drug is taken orally in a prescribed dose during the first seven to nine weeks of pregnancy, and within two days the uterine lining begins to deteriorate, usually causing bleeding similar to that experienced during normal menstruation. The mifepristone is then followed up by a dose (taken orally or as a vaginal suppository) of the synthetic prostaglandin misoprostol, which stimulates the uterus to undergo contractions. The embryo and other uterine contents are expelled in a process very similar to spontaneous abortion, or miscarriage. In a small number of cases the induced abortion is not complete and must be followed by a surgical procedure, most commonly vacuum aspiration. The most common side effects are the usual symptoms of miscarriage-cramping, bleeding, and occasional nausea, dizziness, and back pain. The drug does not reliably terminate pregnancies beyond the early weeks, and it is not prescribed for an ectopic pregnancy (when a fertilized ovum is implanted outside the uterus-for instance, in one of the fallopian tubes)