A sedative commonly used to euthanize animals may be used on death row inmates in Oklahoma to substitute one of the three drugs in the state's lethal injection formula, a federal judge ruled Friday.

U.S. District Judge Stephen Friot rejected a motion by death row inmates Jeffrey David Matthews and John David Duty, who argued that the use of a drug called pentobarbital amounted to "cruel and unusual punishment."

Friot said the inmates' attorneys failed to prove that the new drug posed a "substantial risk of serious harm." The judge said the two anesthesiologists who testified during Friday's daylong hearing agreed that a sufficient dose would render an individual unconscious and ultimately lead to death.

Attorneys for the inmates said an appeal was likely, but declined further comment.

Earlier this year, the Oklahoma Department of Corrections ran out of sodium thiopental, a key component in the three-drug cocktail that causes unconsciousness. The department changed its protocol to allow for the use of pentobarbital, a similar drug.

No other U.S. state uses pentobarbital during executions, experts testified.

"We knew changing drugs was going to be breaking new ground," Assistant Attorney General Stephen Krise said after the judge's ruling. "A lot of effort went into making sure that the drug we chose would satisfy the requirements of the 8th Amendment, and I think that bore fruit today."

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

Dr. David Waisel, an anesthesiologist, testified Friday that pentobarbital is used primarily as an anti-seizure drug or in small doses as a sedative, but not as a general anesthetic.

Substituting pentobarbital increases the risk that an inmate could be paralyzed but aware when the third drug that stops the heart is administered, and 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.

But another anesthesiologist whose videotaped deposition was played in court testified that the amount of pentobarbital the state plans to use in the procedure is enough to cause unconsciousness and even death within minutes.

"Five-thousand milligrams of pentobarbital is an enormous overdose and is much more than would be needed to induce a barbituate coma," Dr. Mark Dershwitz said.

Dershwitz also testified that the risk of an inmate remaining conscious after receiving such a dose is "a probability as low as we could possibly measure."

He said smaller doses of the drug are routinely used to euthanize animals.

Several states that have lethal injection employ a three-drug combination created in the 1970s. Sodium thiopental is injected using a syringe to put an inmate to sleep, then two other drugs — pancuronium bromide, which paralyzes muscles, and potassium chloride, which stops the heart — are administered.

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

Matthews, one of the inmates challenging the new drug, has had his execution 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. The attorney general's office is expected next week to request a new execution date for Matthews.

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. Authorities said 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.