Victorian ambos sometimes use painkiller Fentanyl to deal with job stress - but the paramedics union denies 10 Geelong staff were sacked or disciplined due to the drug.
Ambulance Employees Australia secretary Steve McGhie says about 20 paramedics were interviewed as part of an Independent Broad-based Anti-corruption Commission investigation of Ambulance Victoria.
The Herald Sun reported on Friday five staff had been sacked and five others disciplined following the IBAC investigation into Fentanyl use and "other illicit activities".
But Mr McGhie says the 10 Geelong staff in question resigned or were disciplined for behaviour in their personal or social life that breached the professional code of conduct - not because of the drug.
Mr McGhie says while Fentanyl misuse isn't a "huge issue" among paramedics, he says about six people in recent years have developed a dependency, largely because of undiagnosed mental health injuries incurred at work.
"They've sustained injuries because of the job, post traumatic stress disorder or something like that, and they've been unable to ease their problems and their concerns, and sometimes have resorted to the use of Fentanyl," he told AAP on Friday.
The Herald Sun reported on Friday that at least two paramedics are believed to have died in the past three years by overdosing on the highly addictive drug.
Ambulance Victoria said patient care had not been compromised "by the behaviour of this small group".
"We are confident that no patients have been affected and no patient care compromised by the behaviour of this small group," the statement said.
"We also uphold high clinical and ethical standards at Ambulance Victoria and we will not tolerate people stepping outside these standards."