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TALLAHASSEE, Fla., — An execution scheduled in Florida on Tuesday will be the first using the drug midazolam hydrochloride despite concerns it might not work as promised and could inflict cruel and unusual punishment on a death row inmate.

Midazolam, typically used by doctors for sedation, will be the first of three drugs pumped into William Happ as part of a lethal injection cocktail designed to induce unconsciousness, paralysis and death by cardiac arrest.

The first of the drugs administered as part of the lethal injection "protocol" in Florida has long been the barbiturate pentobarbital. But Florida, and other death penalty states that use a trio of drugs as part of their injection procedures, have been running out of pentobarbital since its manufacturer clamped a ban on its use in future executions.

The 51-year-old Happ, who has abandoned his appeals and said he is ready to die, was condemned for the 1986 abduction, rape and murder of Angie Crowley, whose body was found on a canal bank near Crystal River in central Florida.

"This is somewhat of an experiment on a living human being," Richard Dieter, executive director of the Death Penalty Information Center in Washington, said Monday.

"The three-drug process depends on the first drug rendering the inmate unconscious and, if he is only partially unconscious, the inmate could be experiencing extreme pain," he added. "Because the second drug paralyzes him, he would be unable to cry out or show that he's in pain."

Just last week, Missouri postponed an execution set for Oct. 23 due to uncertainty about using a different drug, propofol, as a substitute for pentobarbital.

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Misty Cash, a spokeswoman for the Florida Department of Corrections, declined to comment on how the state could rest assured that midazolam would avoid inflicting pain and suffering on Happ.

But she said the prison system "did research and determined that this is the most humane and dignified way to do the procedure."

She refused to identify a research laboratory or other source of the department's scientific data, citing an exemption in state public record laws that also meant that the supplier of midazolam hydrochloride for executions could remain anonymous.

"We're not talking about details," she said. "That could impact the safety and security of the process."

Happ's lawyer, Eric Pinkard of St. Petersburg, said there are no late motions to stay the execution. Happ told a circuit judge in Inverness, Florida last month that he did not want to continue the court appeals that have kept him on Florida's Death Row for nearly a quarter-century.

Happ's execution is scheduled for 6 p.m. EDT (2200 GMT) on Tuesday at the Florida State Prison in Starke.

Another condemned Florida prisoner, Etheria Jackson, has a hearing set for a Nov. 6 in Jacksonville's federal court, challenging the use of midazolam.

Jackson's appeal contends that there is "substantial risk" of midazolam not working completely, violating the Eighth Amendment's prohibition of "cruel and unusual punishment" by subjecting the condemned to a painful paralysis and fatal heart seizure over several minutes.