A team of international researchers is warning that assisted suicide can lead to 'inhumane' deaths, with some people taking as long as seven days to die. 

Although one of the main arguments in favour of assisted suicide is alleviating pain and suffering, the researchers said that current techniques were falling short of the 'optimum method' of ensuring a person was unconscious prior to dying in order to minimise pain and distress.

The result of this, they said, was that some people risked being killed by 'suboptimal, or even cruel, means', especially the 'vulnerable'. 

The researchers revealed their findings in a paper published this week in the Anaesthesia journal in which they argued that any decision by a society to sanction assisted dying 'should logically go hand‐in‐hand with defining the acceptable method'. 

The huge inconsistency in methods being used to end people's lives was further evidence that doctors have not yet agreed on the best way to carry out assisted suicide, they said. 

In some US states and European countries, the most common method was a self‐administered barbiturate ingestion, which they said resulted in the person dying 'slowly' from asphyxia. 

In the Netherlands, a physician-administered injection combining a general anaesthetic and neuromuscular blockade are permitted. 

The researchers said the Dutch injection technique was comparable with methods used in capital punishment in the US. 

Their research also uncovered instances of severe discomfort, with a high rate of vomiting (up to 10%) and a prolongation of death, with some people taking up to seven days to die after the drugs had been administered. 

'This raises a concern that some deaths may be inhumane,' they said, adding, 'We found that the very act of defining an "optimum" itself has important implications for ethics and the law.'

There were also some cases of people waking up again from the coma state (up to 4%), while a small number of patients even sat up during the dying process, the authors said.

'Each of these potentially constitutes a failure to achieve unconsciousness, with its own psychological consequences,' they said.

They added, 'Although the ethical dilemmas are widely debated, the precise means to achieve unconsciousness are rarely a focus of discussion.'

The Society for the Protection of Unborn Children said the report undermines the arguments of pro-assisted suicide campaigners that the practice is more humane than a natural death.

'This report tears huge holes in some of the key arguments for assisted suicide,' it said.

'It has been legalised in many countries without an agreed way of ensuring the death is free from suffering. Any way of ensuring the death is pain-free will involve intensive medical input - something many are trying to avoid.'