Justice Samuel A. Alito Jr. wrote the majority opinion in the 5-to-4 decision. He was joined by the court's four more conservative justices.

The drug, the sedative midazolam, played a part in three long and apparently painful executions last year. It was used in an effort to render inmates unconscious before they were injected with other, severely painful drugs.

Four condemned inmates in Oklahoma challenged the use of the drug, saying it did not reliably render the person unconscious and so violated the Eighth Amendment's ban on cruel and unusual punishment. Lower courts disagreed.

Oklahoma and several other states started to use midazolam in executions after manufacturers in Europe and the United States refused to sell them the barbiturates that were traditionally used to produce unconsciousness.