Amnesty International has urged doctors and nurses not to participate in executions by lethal injection as it breaches their ethical oath.
Amnesty International questions the cocktail of drugs used
In a report the group says the cocktail of drugs used is not always quick and painless and can cause "excruciating pain and extreme mental suffering".
The execution method is common in the US and is on the rise in China.
However, the US Supreme Court last week agreed to hear a challenge that lethal injections violate the constitution.
Amnesty opposes the death penalty in all forms.
In its new report, "Execution by lethal injection - a quarter century of state poisoning", it says governments should not put doctors and nurses in the position of carrying out an action contrary to their ethical oath.
Jim Welsh, the group's health and human rights co-ordinator, said: "Medical professionals are trained to work for patients' well-being, not to participate in executions ordered by the state."
The report also challenges the cocktail of three drugs commonly used in executions.
It says that Texas, the biggest US user of lethal injections, has banned the same drugs for dogs and cats on the grounds of the potential pain they may suffer.
US LETHAL INJECTION
Used in 37 of 38 death penalty states. Nebraska uses electric chair
Almost all use same three-drug combination:
Sodium thiopental (sodium pentothal): Induces unconsciousness
Pancuronium bromide (Pavulon): Causes muscle paralysis
Potassium chloride: Stops the heart
Source: Amnesty/Death Penalty Information Center
The group says the drug used to induce unconsciousness can wear off before the prisoner's heart stops, causing extreme physical and mental strain.
The patients are, however, in a "chemical straitjacket" and cannot convey their distress, it says.
Amnesty cites case studies of US prisoners suffering for about 30 minutes in "botched" executions.
It says there are no exact official figures on executions in China but that it is certain to carry out more than any other country.
Amnesty says lethal injections are on the rise in China, with mobile vans increasingly being used.
Prisoners are executed on a metal bed in a windowless chamber in the back, the group says.
The issue of lethal injection is a matter of huge debate in the US.
Last week the Supreme Court agreed to hear a case brought by two convicted murderers in Kentucky who argue that the method violates the ban on cruel and unusual punishment contained in the Eighth Amendment to the US Constitution.
The court ruling may provide a broad guideline on the method of execution, which some states have suspended after claims it was cruel and ineffective.
Andrea Keilen, from a legal firm that represents about 150 death row inmates in Texas, said there was no way of knowing the competency of those carrying out the executions in the state.
"We don't have any idea about what's happening in Texas, because it's done in secret."