The future of a commonly used abortion pill is now before the United States Court of Appeals for the Fifth Circuit, which is poised to consider the availability of a drug used in more than half of abortions across the United States.

The Supreme Court last month temporarily halted steps to sharply curb the distribution of the pill, mifepristone, as an appeal moves forward.

It is the latest development in a tangled and pitched fight that has complicated the legal status of the drug, mifepristone. The case could have wide-ranging consequences even in states where abortion is legal, as well as for the F.D.A.’s regulatory authority over other drugs.

Here’s what is in play now.

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

In April, in a case filed by a consortium of anti-abortion doctors and groups, 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 Food and Drug Administration’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.

That decision essentially turned back the clock seven years, before the F.D.A. made a series of changes in 2016 that eased access to the pill. The restrictions laid out by the Fifth Circuit’s earlier ruling would require 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. Those rules would have effectively prevented patients from receiving mifepristone by mail.

Before the changes in 2016, mifepristone was only authorized for use until seven weeks into pregnancy; now the time frame is 10 weeks, allowing medication abortion for many more patients.

And the appeals court ruling appeared to have declared invalid the F.D.A.’s approval in 2019 of a generic version of mifepristone, manufactured by GenBioPro, whose product now accounts for two-thirds of the mifepristone used in the United States.

Such a ruling 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. And leaders of the pharmaceutical and biotech 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.

The Supreme Court order last month ensured that the pill will remain widely available for now. An appeal in the case is now before the Fifth Circuit, where on Wednesday, a different three-judge panel will hear oral arguments related to Judge Kacsmaryk’s preliminary ruling. Each side has been allotted 40 minutes to make their case.

The appeal process could take months as it winds through the courts.

Any decision by the Fifth Circuit panel is likely to be appealed, and the case could then move before all of the circuit judges. Then, the losing party can appeal to the Supreme Court.

Access to the abortion pill will remain unchanged until the Supreme Court intervenes or declines to hear the case.

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.

The merits of the case are still pending before Judge Kacsmaryk, meaning that the case is still in the early, pre-trial stages. Eventually, that, too, is likely to make its way to the Fifth Circuit and then to the Supreme Court.

The plaintiffs have claimed 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 unusual and that less than 1 percent of patients need hospitalization.

In his preliminary ruling, Judge Kacsmaryk said that the F.D.A. had improperly approved the drug, essentially ordering it off the market. He gave the agency a week to seek emergency relief before his ruling would take effect.

The Biden administration immediately appealed, and a divided three-judge panel of the U.S. Court of Appeals for the Fifth Circuit said that mifepristone could remain available as the lawsuit makes its way through the courts, but that the 2016 restrictions should take effect immediately. The Biden administration then asked the Supreme Court to intervene, and the justices said last month that the status quo should remain in effect for now.

A second case about the abortion pill is proceeding in a federal courtroom in Washington State, after Democratic attorneys general of 17 states and the District of Columbia filed a lawsuit challenging the renewed F.D.A. restrictions on access to mifepristone.

Less than an hour after Judge Kacsmaryk issued his ruling, Judge Thomas O. Rice of the U.S. District Court for the Eastern District of Washington, an Obama appointee, blocked the agency from doing anything to curb the availability of mifepristone in those 17 states and the District of Columbia. Although his order did not affect the entire country, the states in that lawsuit represent a majority of states where abortion remains legal. Drug policy experts said the order essentially had national implications because it would be logistically untenable for the F.D.A. to apply one set of rules about a drug to some states and not others.

Legal experts said the direct conflict between the Washington State case and the Fifth Circuit’s decision to block specific parts of the F.D.A.’s rules for the abortion drug helped ensure the Supreme Court would have to weigh in.

As the dueling cases work their way through the courts, their outcomes will affect the ultimate resolution of the abortion pill conflict. So will the outcome of two other lawsuits that have since been filed: one by GenBioPro, the maker of the generic version of mifepristone, in a federal court in Maryland, and another by several abortion providers in a federal court in Virginia. Both cases were filed against the F.D.A. and are asking the courts to order the agency to protect access to mifepristone.

Adam Liptak contributed reporting.