Depression and personality disorders are the most common diagnoses in psychiatric patients requesting help to die, research has found.
The study, published in the online journal BMJ Open, focused on Belgian patients who had requested euthanasia on the grounds of "unbearable suffering".
Euthanasia has been legal in Belgium since 2002, with life-ending drugs given either by mouth or administered intravenously.
Researchers tracked requests for help to die made by 100 psychiatric patients - 77 women and 23 men aged between 21 and 80 - receiving treatment in outpatient clinics.
The work, in the Dutch-speaking part of the country, took place between 2007 and 2011, with follow-ups on patients made in 2012.
In total, 91 patients had been referred for counselling, with 73 deemed medically unfit to work and 59 living alone.
Ninety had more than one mental health issue, with 58 patients suffering from depression and 50 from personality disorder.
Euthanasia requests were approved for 48 patients, with 35 having the procedure and eight cancelling or delaying it as having the option gave them "peace of mind" to continue living.
By December 2012, 43 of the patients had died - six committed suicide, one woman died after sedation in a psychiatric hospital and another from anorexia.
Dr Lieve Thienpont of the University Hospital, Brussels, said: "This is the first report on a relatively large series of requests for euthanasia from patients whose request is based on psychological suffering due to a psychiatric illness.
"This data draws attention - and deepens our understanding - to the circumstances of a small but severely affected group of psychiatric patients.
"Further research is needed to lead to a better understanding of patients with psychiatric disorders who request euthanasia due to unbearable psychological suffering."
After an initial discussion with a practitioner, 38 of the psychiatric patients required further tests and treatment.
A total of 13 were tested for autistic spectrum disorder, with 12 subsequently diagnosed with Asperger's syndrome.
After this further testing and treatment, euthanasia requests were approved for 17 of the 38 patients. Ten were carried out.
The remaining 62 patients were assessed, with 31 granted help to die by practitioners. A total of 25 were performed - making 35 across the whole study.
Two patients committed suicide before the procedure could be implemented, while 11 decided to postpone or cancel.
Eight said this was because having the option of euthanasia gave them peace of mind, while two withdrew their requests due to strong family resistance.
One could not have the procedure as they were in prison.
Among the 52 patients whose requests were not accepted, 38 withdrew before a decision was reached.
A further eight continued to pursue their requests. Four committed suicide and two died spontaneously.
Dr Tony Van Loon, of the Free University of Brussels, said: "One finding from our study is that eight of the 10 patients who postponed or cancelled the euthanasia procedure explicitly declared that having the option to die by euthanasia gave them enough peace of mind to continue living.
"From the 35 patients who died from euthanasia, the relatives and the doctors performing euthanasia explicitly reported many had a calm and smooth passing.
"Mostly the procedure took place in domestic surroundings with their family and or friends present at the time of the death."
In the surviving 57 patients, nine cases were still in process at the time of the conclusion of the study.
Requests in 48 cases were on hold because the patients were managing with regular, occasional or no therapy.
In 2010 and 2011, 2,086 patients died by euthanasia in Belgium, accounting for 1% of all deaths during that period. Those who were not terminally ill made up less than 10% of that total.
The researchers say there is currently no consensus on what constitutes "unreasonable suffering", or guidelines on how best to deal with euthanasia requests from psychiatric patients.
"Taking into account the ongoing fierce ethical debates, it is essential to develop such guidelines, and translate them into clear and detailed protocols that can be applied in practice," they added.
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