How safe is it to take abortion pills? The F.D.A., the nation’s authority on drug regulation, says that it’s very safe. But the agency’s judgment is the topic of a sweeping challenge that the Supreme Court will hear tomorrow. The case could curtail Americans’ access to mifepristone, the first pill taken in a two-drug regimen for a medication abortion.

Pills now account for most abortions in the United States. Increasingly, people take the medications at home. About 14,000 medication abortions per month are now prescribed online, with pills sent through the mail.

In today’s newsletter, I’ll explain how the suit against the F.D.A. seeks to shut down this form of access and impose other restrictions. A decision in the plaintiff’s favor would change the landscape of abortion not state by state, like the effects of the 2022 ruling that overturned Roe v. Wade, but across the country.

The abortion opponents who sued the government in tomorrow’s case, F.D.A. v. Alliance for Hippocratic Medicine, are frustrated by how common abortion has remained. Since the high court struck down Roe, 16 states have banned or severely restricted the procedure. In those places, some women who would have ended their pregnancies are carrying them to term, as my colleagues Margot Sanger-Katz and Claire Cain Miller have explained. But the total number of abortions across the country has not fallen. It may actually have increased.

That’s in large part because of abortion pills, which the F.D.A. first approved in 2000. “In the six months after the Supreme Court overturned Roe, the rise in the supply of pills outside of clinics significantly made up for the reduction in abortions otherwise,” said Abigail Aiken, a professor of public affairs at the University of Texas at Austin with a new study on the topic. In some states, CVS and Walgreens recently started dispensing mifepristone along with misoprostol, the second abortion pill, in stores with a prescription.