The governor of Oklahoma has signed a law allowing death row prisoners to be executed with nitrogen gas, becoming the first state to adopt the method.
Oklahoma has been forced to halt its use of lethal injections pending a US Supreme Court hearing later this month because of complaints that one of the three drugs used in the process is unsuitable.
Several states have been using a sedative called Midazolam because the drug which has been used previously was withdrawn by its manufacturers, who were angry it was being used to kill prisoners.
A number of executions involving Midazolam have seen prisoners taking longer to die than recommended.
Opponents have claimed it constitutes “cruel and unusual punishment”, forbidden under the US constitution.
Death penalty supporters in Oklahoma proposed the alternative method of nitrogen hypoxia, which causes a loss of consciousness followed by death from lack of oxygen.
Although the death chamber inside Oklahoma State Penitentiary would have to be redesigned, supporters say nitrogen is a cheap and effective method.
Governor Mary Fallin said: “Oklahoma executes murderers whose crimes are especially heinous.
“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.”
Oklahoma already has the electric chair and firing squad available as alternative methods of execution, though has not used them.
Nitrogen will now be the first choice if lethal injection is ruled unconstitutional or the drugs are not available.
Oklahoma faced a legal challenge after a botched lethal injection last year in which an inmate groaned and writhed on the gurney.
Last month Utah approved firing squads as its first alternative to lethal injection.
The last execution in a gas chamber in the United States was in Arizona in 1999.
Gases such as hydrogen cyanide and carbon monoxide have previously been used.
Four states – Arizona, California, Missouri and Wyoming – have retained gas chambers, but have not used them for many years.