Officials in the US state of Oklahoma have executed a death-row inmate with a drug cocktail that includes a sedative typically used to put down animals.
John David Duty, 58, is thought to be the first US prisoner to be executed using the sedative pentobarbital.
Duty, who killed a cellmate in 2001, was pronounced dead at 1818 local time (0018 GMT) at the Oklahoma State Penitentiary.
A shortage of sodium thiopental in the US forced the state to make the change.
A judge's ruling to allow Oklahoma to substitute pentobarbital for sodium thiopental was upheld by a federal appeals court this week.
Sodium thiopental, an anaesthetic, is usually used in the state's lethal injection formula, which also includes drugs that paralyse muscles and stop the heart.
Oklahoma received a dose of sodium thiopental from Arkansas for an execution in October but was unable to obtain more of the drug, Oklahoma Department of Corrections spokesman Jerry Massie said.
Lawyers representing Duty and two other death-row inmates argued during a court hearing in November that use of the sedative could be inhumane and that inmates could be conscious but paralysed when the other drugs were administered.
"No-one who has been put to death has come back and testified about what it felt like," said lawyer Jim Rowan, a board member of the Oklahoma Coalition to Abolish the Death Penalty.
California, Arkansas, Tennessee and Maryland have delayed executions because of protocol changes, which include the use of the new drugs, Death Penalty Information Center executive director Richard Dieter told the Associated Press news agency.
"I think Oklahoma is the only state where this issue has come to a head over a new drug," Mr Dieter said, adding that other state courts wanted more time to review the changes.
Several of the 35 US states that use lethal injections are hunting for alternatives to sodium thiopental after Hospira, the sole US manufacturer of the drug, said new batches would not be available until early 2011.