What Is Mifepristone and How Is It Used?

Mifepristone is used as part of a two-drug regimen to end a pregnancy without surgery. Along with the second drug, misoprostol, it is known as medication abortion, or “the abortion pill” in common parlance.

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 the traditional 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 used 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.

About half of people who get legal abortions in the United States — and three-quarters in Europe — use medication abortion. During the pandemic, it became more common because patients wanted to avoid going to clinics in person, and a change in federal law made it easier for them to get prescriptions via telemedicine.

Medication abortion is also used illicitly by those who live in a place that restricts legal abortion or by those who can’t reach a clinic. The U.S. abortion rate is higher than officially reported by doctors, evidence suggests, because people are ordering pills online. That invisible abortion rate may rise if more states move to ban abortion.