CALGARY -- A U.S. judge has rejected a request from the state of Montana to change one of the drugs used to execute prisoners on death row.

The decision by District Court Judge Jeffrey Sherlock could be good news for Ronald Smith of Red Deer, Alta., who is one of two inmates condemned to die in that state.

Sherlock presided over a hearing last month on whether the sedative pentobarbital, which was being proposed by the state, complies with language in Montana's execution protocol requiring an "ultra-fast-acting barbiturate."

Lawyers from the American Civil Liberties Union argued that the sedative could lead to an "excruciating and terrifying" death.

"This case is not about whether the use of pentobarbital in a lethal injection setting is cruel and unusual or if pentobarbital in the doses contemplated by the State of Montana would produce a painless death," wrote Sherlock in his decision.

"This case is only about whether the drug selected ... meets the legislatively required classification of being an ultra-fast-acting barbiturate.

"The court rules that pentobarbital is not."

Sherlock's decision means it's back to the drawing board for Montana officials, who are now prevented from going ahead with any executions.

"The State of Montana will either need to select a barbiturate that is ultra-fast-acting ... or it will need to modify its statute."

Lethal injection has been the sole method of execution in Montana since 1997. It is the only state that specifies the death penalty must be accomplished by an "ultra-fast-acting" barbiturate.

Ron Waterman, a senior counsel for the American Civil Liberties Union, was happy with the ruling and said the issue will have to go back to the state legislature.

"It would have to be a bill that is introduced at the next legislative session, which is 2017. It would then have to be passed by both houses of the legislature and then signed by the governor," he said.

Waterman said getting both the legislature and the senate to pass a new law would be challenging for the state of Montana and bodes well for his clients.

"It's very good news," Waterman said. "I believe this is a very good outcome and we're very pleased."

Smith, 57, was convicted in 1983 for shooting Harvey Madman Jr. and Thomas Running Rabbit while he was high on drugs and alcohol near East Glacier, Mont.

He had been taking 30 to 40 hits of LSD and consuming between 12 and 18 beers a day at the time. He refused a plea deal that would have seen him avoid death row and spend the rest of his life in prison.

Three weeks later, he pleaded guilty. He asked for and was given a death sentence. Smith later had a change of heart and has been fighting for his life ever since. He has had a number of execution dates set and overturned.