The Supreme Court on Thursday unanimously preserved access to a medication that was used in nearly two-thirds of all abortions in the US last year, in the court's first abortion decision since conservative justices overturned Roe v. Wade two years ago.

The justices ruled that abortion opponents lacked the legal right to sue over the federal Food and Drug Administration's approval of the medication, mifepristone, and the FDA's subsequent actions to ease access to it.

The case had threatened to restrict access to mifepristone across the country, including in states where abortion remains legal.

The high court is separately considering another abortion case, about whether a federal law on emergency treatment at hospitals overrides state abortion bans in rare emergency cases in which a pregnant patient's health is at serious risk.

More than 6 million people have used mifepristone since 2000. Mifepristone blocks the hormone progesterone and primes the uterus to respond to the contraction-causing effect of a second drug, misoprostol. The two-drug regimen has been used to end a pregnancy through 10 weeks gestation.

Health care providers have said that if mifepristone is no longer available or is too hard to obtain, they would switch to using only misoprostol, which is somewhat less effective in ending pregnancies.

President Joe Biden's administration and drug manufacturers had warned that siding with abortion opponents in this case could undermine the FDA's drug approval process beyond the abortion context by inviting judges to second-guess the agency's scientific judgments.

The Democratic administration and New York-based Danco Laboratories, which makes mifepristone, argued that the drug is among the safest the FDA has ever approved.

The abortion opponents argued in court papers that the FDA's decisions in 2016 and 2021 to relax restrictions on getting the drug were unreasonable and “jeopardise women's health across the nation".

The mifepristone case began five months after the Supreme Court overturned Roe. Abortion opponents initially won a sweeping ruling nearly a year ago from US district judge Matthew Kacsmaryk, a Trump nominee in Texas, which would have revoked the drug's approval entirely.

The 5th US Circuit Court of Appeals left intact the FDA's initial approval of mifepristone. But it would reverse changes regulators made in 2016 and 2021 that eased some conditions for administering the drug.