Inside was the to-do list of a troubled 21-year-old prescription drug abuser, who had been allowed to leave the Canberra Hospital despite expressing suicidal thoughts and barely surviving an overdose earlier that day.

The fifth and final reminder to his addled brain read: "Don't get wasted!"

Within three hours of discharge, Mr Fennessy was dead, his body unable to cope with the cocktail of prescription drugs he had ingested.

Mr Fennessy could never have been expected to help himself.

In the two years before he died, various doctors had prescribed him a dangerous mish-mash of powerful medicines, to which he became addicted and continued to access with relative ease by "doctor-shopping".

The doctors who prescribed him drugs often had little idea what else he was taking.

Mr Fennessy was homeless, struggling with mental illness and poor memory, and lived a chaotic lifestyle with no phone or wallet.

On the day of his death, he told a doctor at the hospital he no longer cared whether he died.

It was a comment recorded in his file, but never accessed by the emergency department's mental health clinician, who went on to deem Mr Fennessy not at risk of self-harm.

His mother, Ann Finlay, who worked at the hospital, had rushed to the emergency department after learning her son had just survived an overdose.

She warned the mental health clinician that her son was at serious risk and needed to be kept somewhere overnight.

The young man had overdosed repeatedly and presented to hospital about 10 times in two years.

"I said I had huge concerns about Paul, there were massive risk factors," she told an inquest into her son's death on Wednesday.

As he sat in emergency, Mr Fennessy's eyes were pinpointed and rolling in opposite directions.

He was acting bizarrely and speaking overly loudly; strange behaviour for the introverted young man who began to abuse prescription drugs to feel normal.

The mental health clinician wrote in his notes that Mr Fennessy was likely to present back to the hospital "in a very short time frame [sic]". The clinician knew that the young man planned to go straight to the pharmacy to get methadone.

Yet, in a decision now under coronial scrutiny, Mr Fennessy was allowed to walk out onto the street.

It's something Ms Finlay cannot comprehend.

"What do you need? What do you actually need to say there is a problem?" she asked the court.

"I think Paul's death could have been prevented that day."

There is no documented evidence that a discharge summary or suicide-risk assessment was filled out.

Mr Fennessy told the mental health clinician he was not suicidal, and his problem was deemed to be one of drug-and-alcohol addiction, not mental illness.

He was found not to be at risk of self-harm and not mentally ill, a finding seemingly at odds with the number of anti-psychotics and anti-depressants he had been prescribed.

Hospital staff told the inquest that left them with no power to detain him.

Mr Fennessy went to the pharmacist, where an error resulted in staff giving him his second dose of methadone for the day. The double-dosing should never have occurred.

He then went home to visit his mother, who asked him to leave the house while she went to the police, as she was desperate for someone to help her son.

He walked around the corner, crouched down and overdosed on the prescription drugs that were in his system.

Ms Finlay, represented at the inquest by barrister James Sabharwal, believes the health system comprehensively failed her son. He spent two years in and out of mental health wards, rehabilitation centres and emergency departments.

Ms Finlay said her son was over-prescribed powerful drugs, and the system could not, or would not, deal with his co-morbidities or look at his case holistically.

She believes if the hospital was better at dealing with co-morbidity, or if her son was given somewhere to stay overnight, he would not have died that day.

The circumstances of Mr Fennessy's death were examined in painstaking detail during a three-day inquest in the ACT Coroner's Court this week.

Coroner Margaret Hunter is expected to deliver her findings in April.

If you need help in a crisis, call Lifeline on 13 11 14. For further information about depression contact beyondBlue on 1300 224 636.