Should the execuction of Troy Davis go ahead on Wednesday night as expected, it would be the fourth occasion he has come within a short time of being administered with lethal drugs in as many years.

On 23 September 2008, Davis came within 90 minutes of execution. He was taken off the gurney after the US supreme court intervened.

That was his second execution date. On 16 July 2007 he was granted a stay just one day before he was due to die, and on 24 October 2008, at the third attempt to kill him, he was spared temporarily three days before his execution date.

Experts in death row and its psychological impact on prisoners say that such multiple exposure to imminent judicial death is tantamount to a form of torture. It can induce post-traumatic stress disorder, and human rights campaigners say it should be classified as cruel and unnatural treatment that should be banned, irrespective of the guilt or innocence of the prisoner.

Stuart Grassian, a psychiatrist with extensive experience of treating death row inmates, has had patients who came within hours of death but were later proven to have been innocent. "I have watched what happens to them, and the effects are horrendous. People suffer immeasurably."

One of his patients came close to being executed on the electric chair. "The image of burning up in the chair stayed with him for years afterwards."

Brian Evans, a death row specialist with the US branch of Amnesty, pointed out that under international law, mock executions were considered to be a form of torture. "Troy Davis's treatment was not a mock execution, but it has had the same effect. Especially when he has come within hours of death, and said his final goodbyes – that is certainly similar to torture."

Evans added that the process was also unbearable for the families of victims. "The constant waiting, the many false promises – that's abusive of their rights too."

For many death row prisoners, the prospect of walking the line, possibly several times, proves to be too much. A 2004 study by John Blume of Cornell law school found that 106 of the 822 executions that had then been carried out in the modern era in America involved prisoners who had voluntarily gone to their deaths, eschewing all appeals.

Some see that as a form of judicial suicide.

In the most gruesome cases, prisoners have actually had their executions called off mid-procedure.

In May 1946, Willie Francis, a black boy then aged 17, was put on the electric chair in Louisiana for murdering his white employer the year before. The instrument of death was improperly set up by a drunk prison guard, and Francis screamed out from behind his leather hood: "I'm n-not dying!"

He was taken back to his cell, then executed back on the same electric chair a year later.

On 15 September 2009, Romell Broom was strapped to a gurney awaiting death by lethal injection in Ohio. Executioners struggled to find a useable vein into which to inject the lethal drugs. Broom was reported to wince and grimace, and at one point appeared to be sobbing.

After two hours, the then governor of Ohio, Ted Strickland, called the execution off. Broom was returned to death row, where he remains today.