WASHINGTON — The Supreme Court said Friday evening that the abortion pill mifepristone would remain widely available for now, delaying the potential for an abrupt end to a drug that is used in more than half of abortions in the United States.
The order halted steps that had sought to curb the availability of mifepristone as an appeal moves forward: a ruling from a federal judge in Texas to suspend the drug from the market entirely and another from an appeals court to impose significant barriers on the pill, including blocking access by mail.
The unsigned, one-paragraph order, which came hours before restrictions were set to take effect, marked the second time in a year that the Supreme Court had considered a major effort to sharply curtail access to abortion.
The case could ultimately have profound implications, even for states where abortion is legal, as well as for the F.D.A.’s regulatory authority over other drugs.
If the ruling by the judge in Texas, which revoked the F.D.A.’s approval of the pill after more than two decades, were to stand, it could pave the way for all sorts of challenges to the agency’s approval of other medications and enable medical providers anywhere to contest government policy that might affect a patient.
The Biden administration had asked the Supreme Court to intervene after the U.S. Court of Appeals for the Fifth Circuit let stand a number of restrictions in the Texas ruling, even as it said it would allow the pill to remain on the market.
In Friday’s order, Justices Clarence Thomas and Samuel A. Alito Jr. dissented.
Justice Thomas gave no reasons, but Justice Alito noted that the Fifth Circuit had already narrowed the most far-reaching aspects of the Texas ruling. The F.D.A. and the manufacturer of the branded version of mifepristone, Danco Laboratories, had “not shown that they are likely to suffer irreparable harm” as the case proceeds through the appeals court, he added.
Justice Alito expressed skepticism of the F.D.A.’s claims that “regulatory ‘chaos’” would ensue if the lower court ruling went into effect. In a nod to a competing case filed by Democratic state attorneys general in Washington State, which is seen as a direct challenge to the case in Texas, he accused the F.D.A. of leveraging the court system to carry out “a desired policy while evading both necessary agency procedures and judicial review.”
This is most likely not the final word from the justices. After the Fifth Circuit hears the appeal, the matter is likely to make its way back to the Supreme Court.
None of the justices appointed by President Donald J. Trump publicly dissented.
The court’s decision is, at least temporarily, a victory for the Biden administration.
President Biden welcomed the decision, saying the “administration will continue to defend F.D.A.’s independent, expert authority to review, approve and regulate a wide range of prescription drugs.”
The Texas ruling, he added, “would have undermined F.D.A.’s medical judgment and put women’s health at risk.”
A spokesman for the F.D.A. declined to comment.
The reaction from the plaintiffs — a coalition of anti-abortion groups and several doctors — was muted.
Erik Baptist, senior counsel for Alliance Defending Freedom, a conservative legal organization that represents the coalition, said the battle would continue.
“The F.D.A. must answer for the damage it has caused to the health of countless women and girls and the rule of law by failing to study how dangerous the chemical abortion drug regimen is and unlawfully removing every meaningful safeguard, even allowing for mail-order abortions,” Mr. Baptist said.
After the Supreme Court eliminated a constitutional right to an abortion in June, political and legal battles shifted to medication abortion, a two-drug regimen that is typically used in the first 12 weeks of pregnancy.
The first drug, mifepristone, blocks the reproductive hormone progesterone, and the second, misoprostol, taken one or two days later, prompts contractions and helps the uterus expel its contents.
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.
The case reached the justices after a swift-moving and tangled fight over the pill’s legal status.
In November, the plaintiffs filed a lawsuit in the Amarillo division of the federal court system in Texas, guaranteeing that the case would come before a single judge: Matthew J. Kacsmaryk of the U.S. District Court for the Northern District of Texas.
Judge Kacsmaryk, an appointee of Mr. Trump, is a longtime opponent of abortion and joined the bench after working at First Liberty Institute, a conservative legal group that focuses on issues of religious liberty.
The coalition that brought the suit, the Alliance for Hippocratic Medicine, argued that the F.D.A. had improperly approved the pill in 2000 and that mifepristone is unsafe. The agency has strongly disputed those claims, pointing to studies that show that serious complications are rare and that less than 1 percent of patients need hospitalization.
This month, Judge Kacsmaryk, in a temporary ruling, declared invalid the F.D.A.’s approval of the drug and gave both parties a week to seek emergency relief before the decision took effect.
Less than an hour later, a federal judge in Washington State, Thomas O. Rice, an appointee of President Barack Obama, issued a contradictory ruling in a separate lawsuit over mifepristone. Judge Rice blocked the F.D.A. from limiting the availability of the pill in 17 states and the District of Columbia, which were parties in that suit.
The competing rulings meant that the matter was almost certainly headed to the Supreme Court.
The F.D.A. immediately appealed Judge Kacsmaryk’s decision, and a divided three-judge panel of the Fifth Circuit, in New Orleans, upheld the agency’s approval of the drug, ensuring that mifepristone would remain on the market.
But the panel imposed several barriers to access, siding in part with Judge Kacsmaryk, while the lawsuit moved through the courts. It blocked a series of steps the F.D.A. had taken since 2016 to increase the availability and distribution of the drug, such as allowing it to be sent by mail and to be prescribed by medical providers who are not doctors.
Adam Liptak and Christina Jewett contributed reporting.