The doctor who gave methadone to an inmate at Canberra's jail two days prior to his death has told an inquest that while unusual, it is not impossible the man died from that dose.

The coronial inquest is investigating the death of 25-year-old Steven Freeman at the Alexander Maconochie Centre (AMC) in May last year.

Mr Freeman was found dead in his cell with a toxic amount of methadone in his system.

Doctor Luke Streitberg told the inquest that if the two doses of methadone he had prescribed were all Mr Freeman had ingested, then he "was surprised of that outcome".

Dr Streitberg said that, in his experience, a death was "very unusual and unlikely" after a 30mg dose of methadone.

He said the vast majority of methadone-related deaths had been related to higher doses.

On Monday the inquest heard Mr Freeman convinced prison staff he was an opioid user when there was no evidence that he was.

It also heard even a small amount of methadone given to someone who was not addicted to opiates could be deadly.

In determining whether an inmate was an opioid user, Dr Streitberg said the jail did not conduct urine testing.

He said dip stick urine testing would only show very recent heroin use and laboratory testing involved considerable delays.

Doctors not consulted about changes to methadone program

On Wednesday the inquest was told the ACT Government had made changes to the jail's methadone program in the wake of Mr Freeman's death.

But Dr Streitberg said he and the other doctors who work at the jail were not consulted.

Dr Streitberg said he personally did not agree with the decision to lower the initial methadone dosage to 20mg and that it "caused considerable controversy amongst the doctors".

He added the decision came from administration.

Dr Streitberg noted the adverse effect a lower start dose could have on opioid users because it may not control their desire for continued use of opioids.

He said inmates were much more likely to use other drugs which increased risks such as overdose and the spread of blood-borne diseases.