The court's unsigned ruling was in response to a question from the U.S. Supreme Court, The Oklahoman reported.
Backers of the Oklahoma law say it would not ban the two-drug protocol approved by the U.S. Food and Drug Administration. But the Oklahoma court, which in December upheld a lower court decision that the law is unconstitutional, said in its opinion Tuesday that the law would bar doctors from using the protocol because one of the drugs, misoprostol, was not approved separately for abortion.
State Attorney General E. Scott Pruitt was harshly critical of the court, The New York Times reported.
"The Oklahoma Supreme Court erred in striking down the law," he said in a statement. "We believe they have erred yet again by interpreting the law more broadly than the legislature intended."
Oklahoma and a number of other states have legislated on chemical abortions. While lawmakers who have voted for the laws say they are trying to ensure the safety of women seeking abortions, abortion-rights activists say they have the effect of banning better medical practices developed since the FDA approved RU-486 in combination with misoprostol in 2000.
"Ninety-six percent of medication abortions in the United States are now provided according to a regimen different from the one described in mifepristone's FDA-approved label," the Oklahoma court said.
The U.S. Supreme Court has not dealt with abortion in several years but appears to be moving towards taking a case involving medical abortions.
On Monday, a federal judge in Texas upheld the part of the state's new abortion law that limited doctors to the FDA-approved protocol. The judge struck down a requirement that doctors performing surgical abortions must have admitting privileges at a hospital within 30 miles.