DALLAS (AP) — Texas, the nation's most active death penalty state, announced Tuesday that it would become the latest to switch to single-drug executions amid a drug shortage that has left states scrambling for acceptable alternatives.

The Texas Department of Criminal Justice said it will begin using a single dose of the sedative pentobarbital to carry out death sentences. It had been using that drug in combination with two others, but its supply of one of the other drugs expired.

Texas began using pentobarbital last year after another drug, sodium thiopental, became unavailable when its European supplier bowed to pressure from death penalty opponents and stopped making it. But pentobarbital is now in short supply after its Danish manufacturer said it would try to prevent its use in executions.

An Oklahoma inmate asked a federal court on Tuesday to halt his upcoming execution because that state has only one dose of pentobarbital left. A lawyer for Michael Hooper said Oklahoma has no backup plan if the drug fails to render Hooper unconscious, and that creates a risk of cruel and unusual punishment.

Texas officials said in May that they have enough doses of pentobarbital to carry out 23 executions. No one has been executed in the state since.

In the three-drug cocktail, Texas officials administered 5 grams of the drug — about 3.4 ounces — to render the inmate unconscious, followed by the muscle relaxant pancuronium bromide and potassium chloride to stop the heart. Pancuronium bromide is the drug that expired.

Department of Criminal Justice spokesman Jason Clark said officials didn't expect the dose of pentobarbital needed to change with the new procedure.

Four other states — Arizona, Idaho, Ohio and Washington — have used a single drug to carry out executions, according to the Death Penalty Information Center. Ohio was the first to use just pentobarbital, during a March 2011 execution.

Other states, such as Missouri, plan to use propofol, the anesthetic blamed for Michael Jackson's death, to do single-drug executions.

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Death penalty opponents claim single-drug executions may be less humane. They point to an April execution in Arizona, where an inmate shook for several seconds after receiving a lethal dose of pentobarbital. The drug was used by itself in that case.

Richard Dieter, executive director of the Washington-based Death Penalty Information Center, said three-drug cocktails kill quicker than a single anesthetic like pentobarbital.

"The person still goes to sleep and gradually loses mental capacity and dies, but it may take a slightly longer time," Dieter said. "I think the idea originally was to cause death quickly, but you needed the anesthetic to make those next two drugs painless."

Texas has carried out more executions than any other state, 482 since the state reinstated capital punishment in 1982. Its next scheduled lethal injection is July 18, when Yokamon Hearn is set to die for killing a 23-year-old stockbroker from Plano, north of Dallas, in 1998.

Hearn's lawyer, Richard Burr, said he was studying the switch to a single drug and hadn't decided yet whether to file an objection to it.

Texas has nine executions, including Hearn's, scheduled between now and mid-November. Clark said switching to a single-drug method now will ensure that all can be carried out as planned.

Dieter said Texas' switch might influence other states and provide more evidence for whether a one-drug procedure works better than previous methods.

"Either way, it provides more evidence that this is or is not the way to go," Dieter said. "Everybody thinks of Texas as the leading execution state. It's a question of numbers."

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Associated Press writer Sean Murphy in Oklahoma City contributed to this report.