OKLAHOMA CITY — With U.S. death penalty states scrambling for alternatives to lethal injection amid a shortage of deadly drugs, Oklahoma legislators believe they've found a foolproof and humane method — nitrogen gas hypoxia.

Without a single dissenting vote, the Oklahoma Senate gave final legislative approval Thursday and sent the governor a bill that would allow the new method to be used if lethal injection is ruled unconstitutional or if the deadly drugs become unavailable. Republican Gov. Mary Fallin supports the death penalty, but her spokesman declined to comment on the measure Thursday.

Critics of using nitrogen gas say that one concern is that the method is untested, and some states even ban its use to put animals to sleep.

Executions are on hold in Oklahoma while the U.S. Supreme Court considers whether the state's current three-drug method of lethal injection is constitutional. Oklahoma and other states have been forced to come up with new drugs and new sources for drugs as pharmaceutical companies, many of which are based in Europe, have stopped selling them for executions.

There are no reports of nitrogen gas ever being used to execute humans. But supporters of Oklahoma's plan argue that nitrogen-induced hypoxia — or a lack of oxygen in the blood — is a humane execution method.

"The process is fast and painless," said Oklahoma City Republican Rep. Mike Christian, a former Oklahoma Highway Patrol trooper who wrote the bill. "It's foolproof.

Added benefits, Christian said, are that nitrogen is easy to access and medical experts wouldn't need to be involved in the process.

"There is no way for anti-death penalty activists ... to restrict its supply," Christian added.

But opponents say there's no way to know whether the method is painless and effective.

"It just hasn't been tried, so we don't know," said Rep. Emily Virgin, a Democrat from Norman who opposes the death penalty. "This is all based on some Internet research and a documentary from the BBC."

"I think it's dangerous for the Legislature to fool itself into thinking they've adopted some proven method for executing the condemned in Oklahoma, when in fact they have authorized a highly experimental method of execution that's banned in many jurisdictions for the euthanasia of animals," said Ryan Kiesel, a former Democratic legislator and the executive director of the American Civil Liberties Union of Oklahoma, which opposes the death penalty.

The changes come after a botched execution last year in which Oklahoma was using a new sedative as the first in a three-drug combination. State officials tried to halt the lethal injection after the inmate writhed on the gurney and moaned. He died 43 minutes after the process began.

The problematic execution was blamed on a poorly placed intravenous line and prompted a lawsuit from Oklahoma death row inmates, who argue that the state's new drug combination presents a serious risk of pain and suffering. The U.S. Supreme Court is scheduled to hear arguments later this month.

Under the bill that passed Thursday, lethal injection would remain the state's first choice for executions and nitrogen gas would be its first backup method — ahead of the electric chair, which the state hasn't used since 1966, and a firing squad, which has never been used in Oklahoma.

Other death penalty states also are looking at alternatives to lethal injection. Tennessee passed a law last year to reinstate the electric chair if it can't get lethal drugs, and Utah has reinstated the firing squad as a backup method.