LITTLE ROCK, Ark. (AP) — Gov. Sarah Huckabee Sanders signed legislation Tuesday allowing executions using nitrogen gas, making Arkansas the fifth state to adopt a method that opponents say is unconstitutional.

The measure signed into law by the Republican governor has been promoted by supporters as a way to carry out executions for the first time in eight years. Arkansas currently has 25 people on death row.

Arkansas has not had an execution since 2017, when it put four people to death before a sedative used in its lethal injection protocol expired. The attorney general’s office has said the state has been unable to purchase more lethal injection drugs because of manufacturers’ opposition to their use in executions.

“As a state we have failed to keep our promises to the friends and family of victims to execute those sentenced to death under our laws,” Attorney General Tim Griffin said in a statement. “That ends now. Act 302 gives the state the tools needed to carry out these sentences and deliver justice.”

Under the nitrogen hypoxia execution method, an inmate is forced to breathe the gas and deprived of the oxygen needed to stay alive. Alabama, the first state to use nitrogen gas, has carried out four executions using that method since it began last year.

Three other states — Louisiana, Mississippi and Oklahoma — have laws allowing it but have not employed the method so far. On Friday a federal appeals court said Louisiana could carry out its first nitrogen gas execution, scheduled for Tuesday.

The number of executions in the U.S. last year remained near historic lows, and they happened in a small number of states.

“The decision to use nitrogen suffocation as an execution method is a dangerous and regressive move that puts Arkansas out of step with national trends away from the death penalty,” Megan Bailey, a spokesperson for the American Civil Liberties of Arkansas, said in a statement.

Opponents say the method increases suffering, citing accounts from witnesses to Alabama executions who said inmates gasped and shook during executions. State officials say those are involuntary movements associated with oxygen deprivation.

Arkansas has expanded the secrecy surrounding its execution process in recent years, enacting laws shrouding the source of lethal injection drugs and other details. Critics worry that the state is not required to disclose the type or source of nitrogen gas it would use.

The law takes effect 90 days after the Legislature adjourns, which will happen in April at the earliest.