Mifepristone is a synthetic steroid used in combination with a second drug, misoprostol, to induce early, non-surgical abortions. Mifepristone acts by blocking progesterone, the hormone that sustains early pregnancies.

Misoprostol causes uterine contractions that expel the pregnancy. The experience of a medical abortion is similar to that of a miscarriage. It is a white powder, highly soluble in methanol and only poorly soluble in water, its structural name is 11β-[p-(Dimethylamino)phenyl]-17β-hydroxy-17-(1-propynyl)estra-4,9-dien-3-one. Empirically it is C29H35NO2

It is used in the termination of pregnancy under fifty days from conception. A typical dose is 600 mg orally then either a further 400 mg or an oral prostaglandin analogue after a further 48 hours. It is between 92% and 99% effective, depending on the trial data examined, within four hours of the second dosing. As a stand-alone agent it is around 80% effective. It works by a competitive interaction with both endogenous and exogenous progesterone at receptor sites. It also has slight antiglucocorticoid and antiandrogenic effects in larger doses.

Mifepristone is contraindicated in cases of ectopic pregnancy, adrenal failure, hemorrhagic disorders, anticoagulant or corticosteroid therapy. Side effects include an expected amount of abdominal pain and vaginal bleeding, also there can be nausea, vomiting and fever. Incomplete termination would require further intervention, such as vacuum aspiration. Mifepristone blocks the progesterone receptor, leading to changes in the endometrial blood supply. In contrast, methotrexate inhibits DNA synthesis and primarily affects rapidly dividing cells. In early pregnancy methotrexate interferes with the process of implantation through its effect on trophoblastic tissue. The net effect of mifepristone and methotrexate — detachment of the trophoblast from the uterine decidua — is the same. Mifepristone also softens the cervix to facilitate expulsion of the pregnancy. The prostaglandin analogue misoprostol accelerates the process of expulsion by stimulating uterine contractions as well as softening the cervix.