The future of a commonly used abortion pill is now back on the doorstep of the Supreme Court after an appeals court ruling.

A three-judge panel of the United States Court of Appeals for the Fifth Circuit issued a decision on Tuesday saying that the abortion pill mifepristone should remain legal in the United States but with significant restrictions on patients’ access to it. The Department of Justice quickly said it opposed imposing those restrictions and would ask the Supreme Court to hear the case.

For now, until the Supreme Court weighs in, nothing will change about the availability of mifepristone, which is used in more than half of all abortions in the United States.

The appeals court decision is the latest development in a tangled and pitched fight that could have wide-ranging consequences even in states where abortion is legal, as well as for the Food and Drug Administration’s regulatory authority over other drugs.

Here’s the latest on what is in play.

What’s at stake?

At issue is the availability of mifepristone, the first pill taken in a two-drug regimen for medication abortion. More than five million women in the United States have used mifepristone to terminate their pregnancies, and dozens of other countries have approved the drug for use.

Last year, after the Supreme Court overturned the right to an abortion in the United States, a consortium of anti-abortion doctors and groups filed suit against the F.D.A. seeking to invalidate its approval of mifepristone and have the pill withdrawn from the market. The plaintiffs claim that mifepristone is unsafe and that the agency’s approval process for the drug was flawed.

The F.D.A. has forcefully countered those claims, contending that the drug is very safe and effective. It has cited a series of studies that show that serious complications are rare. The F.D.A. applies a special regulatory framework to mifepristone, meaning that it has been regulated much more strictly and studied more intensively than most other drugs.

What has happened so far in the case?

In April, Judge Matthew J. Kacsmaryk of the Northern District of Texas, a Trump appointee who is a longtime opponent of abortion, issued a preliminary ruling invalidating the F.D.A.’s 23-year-old approval of mifepristone. Days later, a three-judge panel of the U.S. Court of Appeals for the Fifth Circuit, in New Orleans, struck part of Judge Kacsmaryk’s ruling but affirmed major aspects of it, allowing mifepristone to remain on the market but imposing significant restrictions on it.

Soon afterward, the Department of Justice and the company that makes the branded version of mifepristone sought emergency relief from the Supreme Court, which temporarily paused any changes involving mifepristone until another panel of the Fifth Circuit could hear an appeal of Judge Kacsmaryk’s preliminary ruling.

That panel, which included two judges appointed by President Donald J. Trump and one appointed by President George W. Bush, issued its decision on Tuesday. The ruling kept in place the F.D.A.’s original approval of mifepristone in 2000, as well as its approval in 2019 of the generic version of the drug, which now accounts for about two-thirds of the mifepristone sold in the United States.

But the appeals court’s decision essentially turned back the clock seven years, to before the F.D.A. began making a series of regulatory changes in 2016 that significantly broadened access to the pill. The appeals court said the pre-2016 rules should apply today. Those regulations required mifepristone to be prescribed and dispensed only by a doctor and picked up in person by the patient, who would have to visit the doctor three times during the medication abortion process. Reinstating those rules would effectively prevent patients from using telemedicine to obtain a prescription and from receiving mifepristone by mail.

In 2016, the F.D.A. also extended the time frame for mifepristone use, authorizing it until 10 weeks into pregnancy instead of seven weeks. (The appeals court decision would technically reinstate that seven-week limit, but it would have less practical impact because physicians in most states can legally use medical discretion to prescribe mifepristone until 12 weeks into pregnancy, as there is scientific evidence that abortion pills are safe and effective within that time frame.)

What happens next?

The Supreme Court could deny review, leaving in place the appeals court’s ruling, curbing but not eliminating access to the pill. Or it could agree to hear the appeal.

A decision by the Supreme Court could ensure full access to mifepristone, impose restrictions but stop short of sharply curtailing access, or withdraw approval of the drug.

If the justices uphold the appeals court ruling, it could also pave the way for all sorts of challenges to the F.D.A.’s approval of medications. Legal experts said medical providers anywhere in the country might be enabled to challenge government policy that might affect a patient. Leaders of the pharmaceutical and biotechnology industries have filed briefs saying that the case could undermine their businesses by destroying their ability to rely on a single national standard for their products.

Adam Liptak contributed reporting.