In a separate dissent, Justice Stephen Breyer said the time has come for the court to debate whether the death penalty itself is constitutional. Justice Ruth Bader Ginsburg joined Breyer's opinion.
The Supreme Court's involvement in the case began in January with an unusually public disagreement among the justices over executions.
Then, the court refused to block Oklahoma inmate Charles Warner's execution over the objection of the four liberal justices. In a strongly worded dissent for the four, Justice Sonia Sotomayor said, "The questions before us are especially important now, given states' increasing reliance on new and scientifically untested methods of execution."
Eight days later, the justices agreed to hear the case of three other Oklahoma death-row prisoners. It takes just four votes among the nine justices to agree to hear a case, but five votes to place a hold on an execution.
When the case was argued in late April, the justices engaged in unusually combative exchanges about the Oklahoma case and impassioned debate about capital punishment more generally.
Among the conservatives, Justice Alito said death penalty opponents are waging a "guerrilla war" against executions by working to limit the supply of more effective drugs. On the other side, liberal Justice Elena Kagan contended that the way states carry out most executions amounts to having prisoners "burned alive from the inside."
In 2008, the court upheld Kentucky's use of a three-drug execution method that employed a barbiturate as the first drug, intended to render the inmate unconscious.
But because of problems obtaining drugs, no state uses the precise combination at issue in that earlier Supreme Court case.
Four states have used midazolam in executions: Arizona, Florida, Ohio and Oklahoma. Also, Alabama, Arkansas, Louisiana and Virginia allow for midazolam, but they have not used it in executions.
Last April's execution of Clayton Lockett was the first time Oklahoma used midazolam. Lockett writhed on the gurney, moaned and clenched his teeth for several minutes before prison officials tried to halt the process. Lockett died after 43 minutes.
Executions in Arizona and Ohio that used midazolam also went on for longer than expected as the inmates gasped and made other noises before dying.
Meanwhile, the court challenge has prompted Oklahoma to approve nitrogen gas as an alternative death penalty method if lethal injections aren't possible, either because of a court ruling or a drug shortage.
The Associated Press