THE US state of Alabama says that it’s switching a key drug used in lethal injections earlier than expected, a move that will be challenged by a condemned inmate scheduled to die in less than a month.

Like several other states, Alabama has turned over its supply of sodium thiopental to the Drug Enforcement Administration after questions were raised about how and where the states received the drug.

The drug pentobarbital will now be used as part of the state’s three-drug execution cocktail instead of sodium thiopental, Alabama prisons spokesman Brian Corbett said.

The change comes after attorneys for death row inmate Jason Oric Williams wrote to US Attorney General Eric Holder, asking the federal government to investigate whether Alabama’s supply of sodium thiopental was illegally obtained from Tennessee. That states supply of the drug has also been seized by the DEA.

At least 10 states have switched to pentobarbital or are considering a switch as part of their three-drug methods because of a nationwide shortage of sodium thiopental, a sedative that states used for more than three decades until its only US manufacturer stopped making it in 2009 and then dropped plans to resume production earlier this year.

Alabama has used sodium thiopental since switching from the electric chair in 2002.

An attorney for Williams said he will ask the courts to stop his client’s May 19 execution because the state is changing the drug.

“The state should not be able to make up on the fly how it is going to carry out executions,” Bryan Stevenson said.

He claims pentobarbital works differently from sodium thiopental.

Williams was sentenced to death for killing four people during a 1992 shooting spree in Mobile County.