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 that 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.

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

Opponents say there’s no way to know whether the method is painless and effective.