OKLAHOMA CITY — Oklahoma became the first U.S. state to approve nitrogen gas for executions under a measure Gov. Mary Fallin signed into law Friday, which provides an alternative death penalty method if lethal injections aren't possible either because of a court ruling or a drug shortage.

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. Supporters of the new law maintain nitrogen-induced hypoxia is a humane and painless method of execution that requires no medical expertise to perform.

See also: Oklahoma Republicans push for gas chamber executions

There are no reports of nitrogen gas ever being used to execute humans, and critics say that one concern is the method is untested. Some states even ban its use to put animals to sleep.

"Oklahoma executes murderers whose crimes are especially heinous," Fallin said in a statement announcing that she had signed the bill into law. "I support that policy, and I believe capital punishment must be performed effectively and without cruelty. The bill I signed today gives the state of Oklahoma another death penalty option that meets that standard."

The bill, authored by Republican Rep. Mike Christian and Republican Sen. Anthony Sykes, had passed the state House on an 85-10 vote and cleared the Senate on a 41-0 vote.

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 Christian, a former Oklahoma Highway Patrol trooper who wrote the bill. "It's foolproof."

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.

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. Witnesses at the scene said he called out “man” and “something’s wrong" after initially being declared unconscious. The inmate died 43 minutes after the process began.

Adam Leathers, spokesman for the Oklahoma Coalition to Abolish the Death Penalty, released a statement blasting the new law.

"This is not only a grotesque waste of resources but indicative of a corrupt value system," he said. "It is sad to know that our state's collective bloodlust is so unabated that our leadership feels the need to spend resources to put a back up system into place so state-sponsored murder can go on uninterrupted."

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 new law, 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.

Additional reporting by Mashable