A third execution by lethal injection has gone awry in six months, renewing debate in the United States over whether there is a foolproof way for the government to humanely kill condemned criminals, and whether it's even worth looking for one.

Death penalty opponents say any killing is an unnecessarily cruel punishment. Proponents may favour the most humane execution method possible, but many reject the idea that a few minutes or hours of suffering by a criminal who caused great suffering to others should send government back to the drawing board.

Thirty years ago, states and the federal government gave little thought to the condemned inmates’ comfort. Most executioners used electric chairs, but death row inmates were also hanged, put to death in the gas chamber or faced a firing squad.

Mistakes occurred. Inmates appeared to suffer in the gas chamber. Electric chairs caught fire or malfunctioned and didn't kill. So a growing number of law enforcement officials, legislators and advocates began searching for a foolproof, constitutional method for executions.

In 1977, an Oklahoma medical director appeared to have found a solution. Dr Jay Chapman came up with a three-drug combination that promised to put the inmate to sleep before painlessly and quickly drifting off to death. Chapman's formula replaced the state's use of the electric chair.

Now, calls are mounting to scrap lethal injection, even by those who support capital punishment like Chief Judge Alex Kozinski of the 9th US Circuit Court of Appeals. He believes a completely humane method of execution isn't possible and favours firing squads.

"If we as a society cannot stomach the splatter from an execution carried out by a firing squad, then we shouldn't be carrying out executions at all," Kozinski wrote.

Chapman's three-drug combination became the default execution method for the federal government and in every state — some three dozen — that has capital punishment. Lethal injection was embraced as the best possible way to execute and the apparent painless and swift death it caused were seen as attributes to counter lawsuits and protests that claimed capital punishment was cruel and unusual.

Since then, more than 1,200 inmates have been executed by lethal injection. The US Supreme Court in 2008 ruled the method constitutional. The US Constitution bars cruel and unusual punishment.

"Execution by lethal injection should be a humane way to die," said Arthur Caplan, head of medical ethics at New York University's Langone Medical Center. "But it isn't."

Caplan said that there hasn't been any reported trouble with physicians who in some states can legally help people commit suicide. "So we know it can be done painlessly," he said.

But medical ethicists and professional licensing boards for doctors and nurses forbid their participation in executions, which are carried out by lay workers who sometimes struggle with administering a lethal injection.

Further, pharmaceutical companies — particularly those based in Europe — are refusing to ship prisons the three drugs necessary to mimic Chapman's mixture. That has caused prison officials to scramble to find alternative drugs that may not kill as quickly.

So the lawsuits and protests persist. So do the problems.