Two death row inmates in Oklahoma asked a federal judge on Friday to stop their pending executions after the state said it planned to substitute the anesthetic used during lethal injections because of a drug shortage.

Oklahoma is one of several states that have been scrambling after the sole U.S. manufacturer of sodium thiopental — an anesthetic that renders the condemned inmate unconscious — said new batches of the barbiturate would not be available until January at the earliest. Hospira Inc. has blamed the shortage on problems with its raw-material suppliers.

The Oklahoma Department of Corrections ran out of that drug earlier this year, and instead, the state plans to substitute a similar drug called pentobarbital, which is used in animal euthanasia.

Death row inmates Jeffrey David Matthews and John David Duty have challenged the use of the new drug, alleging it would violate their constitutional rights to be free from "cruel and unusual punishment."

An anesthesiologist testified on Friday that pentobarbital is used primarily as an anti-seizure drug or in small doses as a sedative but not as a general anesthetic. Dr. David Waisel said substituting pentobarbital increases the risk that an inmate could be paralyzed but aware when the third drug that stops the heart is administered. The third drug can cause a 'very painful, very burning sensation," he said.

"We're in unknown territory, so we have to extrapolate, and often when we extrapolate, we are wrong," Waisel testified.

Several states that have lethal injection employ the three-drug combination that was created in the 1970s: First, sodium thiopental is given by syringe to put the inmate to sleep. Then two other drugs are administered: pancuronium bromide, which paralyzes muscles, and potassium chloride, which stops the heart.

Sodium thiopental is a barbiturate, used primarily to anesthetize surgical patients and induce medical comas. It is also used to help terminally ill people commit suicide and sometimes to euthanize animals.

Matthews' execution has been delayed several times because of his objection to substituting sodium thiopental. Matthews was convicted in the 1994 murder of his 77-year-old great-uncle, Otis Earl Short, during a robbery of Short's home.

Duty, whose execution is scheduled for Dec. 16, was convicted of the December 2001 murder of 22-year-old Curtis Wise, who was Duty's cellmate at the Oklahoma State Penitentiary in McAlester. Wise was strangled to death with shoelaces. At the time, Duty was serving three life sentences for rape, robbery and shooting with intent to kill, all dating from 1978.