If the Supreme Court sides with the plaintiffs and decides to roll back or invalidate F.D.A. regulations on mifepristone, it would be the first time the court undercut the federal agency’s authority to independently decide the conditions governing when and how a drug can be used.

Experts on drug policy and hundreds of pharmaceutical companies have raised alarm at that prospect. Such a ruling could spur lawsuits over other drugs, they say, including those that have been politically divisive, such as vaccines or morning-after pills. Companies say that if they cannot rely on the F.D.A.’s regulatory authority, it could affect their decisions about which drugs to develop and how to make them available.

“The resulting litigation and regulatory uncertainty could destabilize the drug-approval process, undercut drug development and investment, chill innovation, and compromise patient health,” an amicus brief filed by more than 300 leaders of companies and industry associations said.

The lawsuit, filed in November 2022, sought to overturn the approval of mifepristone. Last 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 to pull the medication off the market. Later, a panel of the U.S. Court of Appeals for the Fifth Circuit kept the initial drug approval in place but rolled back changes the F.D.A. made in 2016 and 2021 that increased the accessibility of mifepristone to for patients.

If the Supreme Court sides with the Fifth Circuit and orders that mifepristone be subject to pre-2016 regulations, patients would no longer be able to obtain a prescription by telemedicine and receive abortion pills in the mail. They would have to pick up mifepristone in person from a doctor and would have to visit the doctor three times during the medication abortion process. Nurse practitioners, nurse midwives and physician assistants would no longer be able to prescribe mifepristone, further limiting patients’ access.

“If allowed to stand, the Fifth Circuit’s approach could render drug development unworkable and freeze approved conditions of us in time, depriving patients of the benefits of evolving science and imposing outdated, unnecessary burdens on health care providers,” the pharmaceutical industry’s amicus brief said.

“Far from being limited to a single drug,” the brief said, such a decision “will create chaos in the processes for drug development, approval, and modification.”

The impact of the Supreme Court’s decision depends on how broadly it rules. For example, some legal experts say the F.D.A. might be able to keep the 2016 and 2021 regulations in place while it gathers additional data for those regulations in way that would pass muster with the court. In addition, since a ruling would apply to the F.D.A. and not to abortion providers, some medication abortion services have been stockpiling mifepristone and may continue prescribing and mailing their supply.

If providers could not obtain mifepristone, many would prescribe only the second drug in the medication abortion regimen, misoprostol, which is used for other medical conditions and is widely available by prescription. It is used on its own in many countries, although it is considered somewhat less effective and more prone to causing uncomfortable side effects like nausea, chills and diarrhea.