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 last year found that patients are generally satisfied with it.

Growing evidence from overseas 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 will not be affected by any 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 patients wanted to avoid going to clinics in person, and a change in federal regulation made it easier for them to get prescriptions via telemedicine and to fill them in a pharmacy.

Since nearly two dozen states banned or restricted abortion following the Supreme Court’s Dobbs ruling, some women in those states have turned to mail-order abortion pills instead. Clinicians in several states have begun mailing pills into states with bans, protected by so-called shield laws. And foreign nonprofit groups and entrepreneurs have shipped some women pills from overseas. A court decision restricting mifepristone would affect providers in all states, including those where abortion remains legal. But it would not influence the availability of the overseas pills, which operate outside the bounds of the legal U.S. health care system.