Mr Nichol died after receiving the heroin substitute methadone
Staff at a rehab clinic were warned that a patient may have had too much methadone hours before he died from an overdose, an inquiry has heard.
Kieran Nichol, 20, from Hawick, died from a methadone overdose in the Castle Craig clinic, near Peebles, in the Scottish Borders, in December 2005.
His parents believe staff gave him methadone despite claiming that Mr Nichol was not addicted to heroin.
But an ex-patient said Mr Nichol had told him he was a heroin addict.
On Thursday, the inquiry heard from fellow patient Ross Dixon who claimed he had raised concerns to a nurse about Mr Nichol's methadone dosage on 10 December, the day before he died.
He said: "We had some kind of exchange when I was in and something was said along the lines of Kieran is in a bit of a state, is it possible that he has been given too much methadone.
"The nurse said yes."
But, Mr Dixon claimed, despite warnings, staff did not appear concerned.
"I was under the impression that the facility was aware of his condition," he said.
"Everyone saw what he was like at the AA meeting and I was under the impression it was under control."
Later that evening Mr Nichol passed out during a showing of a film in the communal room of the clinic and died soon after.
Mr Dixon, a former heroin addict, said he was "shocked and upset" that a person could die in a rehabilitation facility from an overdose.
He said: "At every meal he became increasingly intoxicated after being given his medication.
"I found it strange that he was being given methadone when he didn't need any.
"He didn't need any at all, quite the opposite."
Mr Dixon claimed that Mr Nichol had confessed to him that he had had a £280-a-week heroin habit in the weeks leading up to his admission to Castle Craig.
He said: "He said he was in for using heroin."
The inquiry also heard from Mr Nichol's mother, Jacqueline, who told the hearing that her son did not have a heroin problem.
Mrs Nichol said her son had been admitted to the £400-a-day hospital in Blyth Bridge in a bid to cure his depression and addiction to valium.
He spent six weeks "detoxing" before discharging himself in October 2005, the hearing heard.
Two months later, he lapsed back into depression and drug use, and was readmitted two days before he died.
"To be told less than 48 hours later he's died in their care, yes we were shocked," said Mrs Nichol.
The fatal accident inquiry at Edinburgh Sheriff Court continues.