Doctors were forced to allow a suicidal woman who had swallowed anti-freeze to die, because she refused medical help.
Kerrie Wooltorton, 26, of Norwich, had also made a "living will" requesting no intervention if she tried to take her own life, a Norwich inquest heard.
Doctors would have risked breaking the law by treating her, the coroner said.
The Norfolk and Norwich University Hospital said Miss Wooltorton was conscious and doctors were convinced of her mental competence.
She died four days after being admitted to the hospital.
I could have forced treatment on her, but I don't think it was the right thing to do - I feel it would have been assault
Alexander Heaton Consultant renal physician
The hospital said the living will did not play a part in the decision.
Miss Wooltorton had accepted lifesaving dialysis at the hospital after drinking anti-freeze several times in the year before her death, an earlier hearing was told.
"Any treatment... in the absence of her consent would have been unlawful," said coroner William Armstrong.
The inquest was told Miss Wooltorton had mental capacity and had the right to refuse medical intervention.
"Even when she was losing consciousness she was absolutely clear in refusing treatment," said Mr Armstrong.
At the opening of the inquest in October 2008, it was said that Miss Wooltorton had an "untreatable" emotionally unstable personality disorder.
Miss Wooltorton did not want to die alone, the inquest was told
Consultant renal physician Alexander Heaton said his team had monitored Miss Wooltorton for any signs of ambivalence in her decision when she was admitted to hospital on 18 September, 2007.
"It's my duty to follow her wishes," he told the inquest.
"I would have been breaking the law and I wasn't worried about her suing me.
"But I think she would have asked, 'What do I have to do to tell you what my wishes are?'.
"She had made them abundantly clear and I was content that that was the case."
He added: "She was in no state to resist me and I could have forced treatment on her, but I don't think it was the right thing to do. I feel it would have been assault."
Her written directive also stated that if she called an ambulance it was not a plea for help but because she did not want to die alone and in pain, the hearing was told.
Speaking at the resumption of the inquest, Mr Armstrong said: "The doctor went over and above what was required of him.
She had capacity to consent to treatment which, it is more likely than not, would have prevented her death.
Coroner William Armstrong
"He discussed the case with clinical colleagues, took a second opinion from a fellow consultant and sought advice from the medical director.
"A deliberate decision to die may appear repugnant, but any treatment to have saved Kerrie's life in the absence of her consent would have been unlawful."
Recording a narrative verdict, he said Miss Wooltorton had died as a result of "deliberately consuming a poisonous substance in the full knowledge that death could result".
"She had capacity to consent to treatment which, it is more likely than not, would have prevented her death.
"She refused that treatment in full knowledge of the consequences and died as a result," he said.
Local MP and Lib Dem spokesman on health Norman Lamb said: "Doctors need to be given clear guidance on whether they are right to respect the wishes of the individual."
The British Medical Association's head of ethics and science, Dr Vivienne Nathanson, said: "It is always very tragic if an individual, like Kerrie Wooltorton, decides they do not want to live any longer.
"However, patients who are mentally competent must retain the legal right to refuse medical intervention."
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