The state of Florida is set to execute an inmate using the untried compound midazolam hydrochloride as a substitute for the conventional barbiturate that was recently made unavailable by European manufacturers opposing the role of their products in U.S. death penalty protocols. The constitutionality of the state's decision has now been challenged by critics who claim that the use of untried substances in lethal injection cocktails constitutes cruel and unusual punishment.

U.S. lethal injections typically involve three different compounds administered in sequential order: a barbiturate that induces unconsciousness, a paralytic, and potassium chloride, which stops the inmate’s heart. But with chemical supplies diminishing the face of sales bans, Florida has made the decision to substitute the barbiturate pentobarbital with midazolam hydrochloride, an agent commonly used by doctors for sedation. Now, opponents of capital punishment are voicing concerns that an “experimental” drug may bring needless pain and suffering.

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

“Because the second drug paralyzes him, he would be unable to cry out or show that he's in pain,” he added.

The inmate in question is 51-year-old William Happ, who was sentenced to death for the brutal murder of 21-year-old Angie Crowley near Crystal River, Fla. in 1986. After smashing the young woman’s car window and dragging her out of the vehicle, Happ took her to a secluded area where he beat her, raped her, and strangled her with her pants, the Associated Press reports. He has now abandoned his appeals process, and is scheduled to die at 4 p.m. EDT on Tuesday Oct. 15.

While the state has acknowledged the critics’ concerns, officials maintain that the new compound has been selected with great care. Misty Cash, a spokeswoman for the Florida Department of Corrections, told reporters that the prison system "did research and determined that this is the most humane and dignified way to do the procedure." However, she declined to offer any further comments.

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

Florida is not the only death penalty state affected by the sales ban. In August, the state of Texas announced that its own supply of pentobarbital would expire within a month. Still, the state will move forward with the six executions it has slated for the remainder of the year.