OKLAHOMA CITY (AP) – A death row inmate asked a federal court on Tuesday to halt his execution next month because Oklahoma has only one dose left of a lethal injection drug, and it might not work.

Pentobarbital has become hard to get after its manufacturer stopped selling it for executions last year.

An attorney for Michael Hooper said in a motion filed Tuesday that with one remaining dose, Oklahoma has no backup plan if the drug fails to render Hooper unconscious.

The motion mentions cases where anesthesia drugs “failed to take hold” but doesn’t give specifics.

Concerns were raised in April after an Arizona inmate shook for several seconds upon receiving a lethal dose of pentobarbital. In that case, the drug was used by itself. In Oklahoma, it is used with two other drugs.

Other states have switched to other drugs, including propofol, the anesthetic blamed for Michael Jackson’s death, to do single-drug executions.

“The (Oklahoma Department of Corrections) protocol is outmoded and, compared to the one-drug protocol now employed by four other states in over 20 executions, creates a risk of cruel and unusual punishment which is now unacceptable,” attorney Jim Drummond wrote in the motion.

Oklahoma switched to pentobarbital in 2010 as the anesthetic in its three-drug combination. But it has only one 5-milligram dose of pentobarbital remaining and is one of several states facing a shortage after the drug’s Danish manufacturer, Lundbeck Inc., restricted its use last year.

In Oklahoma’s three-drug method, pentobarbital is injected to render an inmate unconscious, followed by vecuronium bromide to stop the breathing and potassium chloride to stop the heart.

Hooper, 39, is scheduled for execution on Aug. 14 for the 1993 killings of his ex-girlfriend, 23-year-old Cynthia Lynn Jarman, and her two children, 5-year-old Tonya and 3-year-old Timmy. Each of the victims was shot twice in the head, and their bodies were buried in a shallow grave in a field northwest of Oklahoma City.

Hooper declined an opportunity earlier this month to seek clemency from the state’s Pardon and Parole Board.

“We’re not litigating his guilt, and we’re not asking the governor or the pardon and parole board to grant clemency,” Drummond told The Associated Press. “He wants a humane death, and we feel that the risks here are substantial.”

Diane Clay, a spokeswoman for Oklahoma Attorney General Scott Pruitt, said his office had not yet been formally notified of the filing and she couldn’t comment on pending litigation.

A spokesman for the Oklahoma Department of Corrections said it hadn’t received any reports of problems with pentobarbital since Oklahoma became the first to use the drug in executions in 2010. It has since been used in more than a dozen states.

Texas announced Tuesday that it was switching to a single-drug execution method using pentobarbital.

“It’s never not worked,” Oklahoma prisons spokesman Jerry Massie said. “If he’s not unconscious, the other two drugs will not be administered. But that’s never happened before.”