What is mifepristone and how is it used?

Mifepristone is one of two drugs used in a medication abortion. It is combined with a second drug, misoprostol, to end a pregnancy.

Mifepristone blocks a hormone called progesterone that is necessary for a pregnancy to continue. Misoprostol brings on uterine contractions, causing the body to expel the pregnancy as in a miscarriage.

In U.S. studies, the combination of these pills causes a complete abortion in more than 99 percent of patients, and is as safe as an abortion procedure administered by a doctor in a clinic. A variety of research has found that medication abortion has low rates of adverse events, and a study published in The Lancet in 2022 found that patients are generally satisfied with it.

Growing evidence from outside the United States suggests that abortion pills are safe even among women who do not have a doctor to advise them.

While the only F.D.A.-approved method in the United States is to use both pills, misoprostol can also end a pregnancy when used alone, and its availability was not affected by the ruling on mifepristone. Misoprostol — which is around 80 percent effective on its own, although it sometimes has to be taken more than once — is also prescribed to treat ulcers, and is available over the counter in many countries, including Mexico.

The Food and Drug Administration has approved medication abortion for up to 10 weeks of pregnancy, while World Health Organization guidelines say it can be used up to 12 weeks at home and after 12 weeks in a medical office. The vast majority of abortions occur before 12 weeks.

More than half of people who get legal abortions in the United States — and three-quarters in Europe — use medication abortion. During the Covid-19 pandemic, it became more common because