US: American researchers have called for an halt to lethal injection, the most common method of capital punishment in the United States, because it is not always a humane and painless way to die.
Some executed prisoners may have suffered unnecessarily because they had not been sedated properly, they said.
The way inmates are given lethal injections does not meet veterinary standards for putting down animals, they added.
The research was carried out by Leonidas Koniaris of the University of Miami Miller School of Medicine, and colleagues.
"Failures in protocol design, implementation, monitoring and review might have led to unnecessary suffering of at least some of those executed," Dr Koniaris said in a study published in the Lancet medical journal.
"Therefore, to prevent unnecessary cruelty and suffering, cessation and public review of lethal injections is warranted."
Anaesthesia is given during a lethal injection to minimise suffering. Without it the prisoner would suffocate and experience horrible pain, according to Dr Koniaris.
But in their analysis of protocols followed during lethal injections in Texas and Virginia, where 45 per cent of executions in the US are conducted, they found there was no monitoring of the anaesthesia.
Emergency medical technicians who administered the drugs had no training in anaesthesia.
When the researchers examined data from autopsies done following 49 executions in Arizona, Georgia and North and South Carolina, they found concentrations of the drug in the blood in 43 cases were lower than that needed for surgery.
Twenty-one prisoners had drug levels that were consistent with awareness.
"We suggest it is possible that some of these inmates were fully aware during their executions," said Dr Koniaris.