Indigenous man Steven Freeman took part in a practice in which prisoners in a methadone program at the jail where he died would swallow the drug, then later vomit it up to share with other inmates, a coronial inquest has heard.
Freeman was found dead in his cell in 2016, after his second day on the methadone program at Alexander Maconochie Centre.
On Monday the ACT Coroner's Court was told Freeman had been a known user of cannabis and methamphetamine, but did not have a history of opioid use.
Akey issue in the inquiry into his death is how he came to be admitted to the program, when he was not an opioid user.
Forensic pathologist Johan Duflou told the court he found no needle marks on Freeman's body and there was no evidence he had used other drugs in jail.
He said Freeman died after inhaling fluid, possibly from vomiting, after his cough response and respiration was seriously depressed by the methadone in his system.
"They do not stop breathing, so much as stopping being able to respire effectively," he said.
The court also heard Mr Freeman told another prisoner he had tried methadone several times earlier, in so-called "drinks" given by other prisoners.
The court was told prisoners would get their dose of methadone, and swallow water afterwards in front of staff, before vomiting up the liquid and sharing it with other prisoners.
Freeman 'straighty-one-eighty', cellmate says
Freeman's cellmate Jermaine Goolagong told the court he tried to talk him out of entering the methadone program.
"He said to me 'I just want to get high'," he said.
He also told the inquest he was surprised at Freeman's dose.
"They gave him a regular junkie's dose," he said.
Mr Goolagong also said Freeman fell asleep listening to loud music on the night he died, and snored very loudly.
"All the nights I have been there I have never heard him snore like that," he said.
Mr Goolagong said he was able to wake Freeman during the night and speak to him, but in the morning he saw froth on his mouth, and he did not respond.
He also denied Freeman used other drugs in jail.
"He was straighty-one-eighty," he said.
'Methadone sensitivity differs between people'
Professor Duflou told the court methadone depressed the central nervous system and reduces the cough response, or the method of removing material from the lungs.
He said the result was a type of pneumonia.
"The aspiration pneumonia, which caused the death, was the result of methadone toxicity," he said.
He said there were risks with methadone prescription, because the differences between therapeutic, toxic and fatal amounts could differ from person to person.
"Methadone is a difficult drug to assess in drug levels and their effect," he said.
Professor Duflou said Freeman appeared to be someone who was particularly sensitive to the drug.
Freeman's family has been in court for the hearing, sometimes appearing distressed by the evidence, as each witness was quizzed about the case.