The court, in a 5-4 decision, handed a loss to three inmates who objected to the use of a sedative called midazolam, saying it cannot achieve the level of unconsciousness required for surgery, making it unsuitable for executions.

Justice Samuel Alito wrote on behalf of the court's conservative majority that the inmates had, among other things, failed to show that there was an alternative method of execution available that would be less painful.

In a dissenting opinion, liberal Justice Stephen Breyer said the court should consider whether the death penalty itself is constitutional. He was joined by one of his colleagues, Justice Ruth Bader Ginsburg.

The three-drug process used by Oklahoma prison officials has been under scrutiny since the April 2014 botched execution of convicted murderer Clayton Lockett. He could be seen twisting on the gurney after death chamber staff failed to place the intravenous line properly.

Inmates Richard Glossip, John Grant and Benjamin Cole challenged the procedure. Glossip arranged for his employer to be beaten to death. Grant stabbed a correctional worker to death. Cole killed his 9-month-old daughter.

The main question before the nine justices was whether the use of midazolam violates the Constitution's Eighth Amendment prohibition on cruel and unusual punishment.

The case did not address the constitutionality of the death penalty in general, but it brought fresh attention to the ongoing debate over whether the death penalty should continue in the United States at a time when most developed countries have abandoned it. During the oral argument in April, conservative Justice Samuel Alito said the challenge to the drug was part of a "guerrilla war" against the death penalty.