A prisoner on death row in the US state of Oklahoma for killing his cellmate in 2001 is due to be executed using a drug cocktail that includes a sedative typically used to euthanize animals.
John David Duty, 58, is set to become the first US inmate to be executed using the sedative pentobarbital.
He is scheduled to die at 1800 local time (0000 GMT) at the Oklahoma State Penitentiary in the town of McAlester.
A shortage of sodium thiopental in the US forced the state 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.
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.
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. BBC News
