Nicholas Brown, Matthew Winwood, Toni Wiki and Belinda Kemp, who were all drug dependent, died in Tasmania between September 2016 and August 2017.
They were being treated by David Jackson, who prescribed them methadone and benzodiazepines as part of opioid replacement therapy.
Mr Brown, 35, died of combined methadone and benzodiazepine intoxication and Mr Winwood, 47, died of mixed prescription drug toxicity involving methadone and multiple sedatives.
Coroner Olivia McTaggart found Dr Jackson put Mr Brown, a lifelong drug user, in danger by prescribing methadone on three occasions without verifying his recent prison prescriptions.
Dr Jackson provided "effectively an unlimited supply for a drug binge" to Mr Winwood, who was clearly not stable and despite warnings from his mother, Ms McTaggart said.
"He may have died by drug overdose eventually, such was his high risk, but that is not to the point," she said.
"He was deprived of a chance to live at that time."
Dr Jackson had a "dangerous manner of treatment and prescribing" in respect of Mr Brown, Mr Winwood, Ms Wiki and Ms Kemp.
"The actions of Dr Jackson, by his grossly irresponsible prescribing, played a direct causative role in the deaths of Mr Brown and Mr Winwood," Ms McTaggart said.
"He did not play a direct role in the deaths of Ms Wiki or Ms Kemp."
Dr Jackson, who worked in Tasmania from 1986 to 2018, was the subject of a large-scale criminal investigation as a result of the deaths but wasn't charged.
Tasmania's Director of Public Prosecutions in 2021 determined evidence could not sustain convictions for manslaughter, even if Dr Jackson's prescribing was dangerous or negligent or contrary to appropriate clinical guidelines.
In 1992, Dr Jackson came to the attention of senior health staff at a Hobart hospital who had significant concerns about his prescribing of excessive opioids to patients, Ms McTaggart said.
In 1995, he again came to the attention of the chief pharmacist relating to excessive prescribing of methadone.
In 2007, Dr Jackson's prescribing practices came under scrutiny from a colleague in relation to a "known problem" with drug-addicted patients attending a health centre where they both worked.
The colleague told the inquiry Dr Jackson kept limited notes in relation to treating his patients which made it hard to understand his logic behind prescribing high doses.
Dr Jackson was prohibited by the national regulator from prescribing certain drugs in January 2018 after a notification from investigating police. He resigned later that month.
Ms McTaggart made 10 recommendations for reform, including for Tasmania's health department to set up a robust strategy to refer breaches by prescribers for prosecution.
Ms Wiki was ruled to have died of cardiac arrest, while Ms Kemp died of pneumonia with mixed drug toxicity and other issues contributing.
