Volunteer first-aiders employed by Ambulance Victoria are being trained to administer a controlled drug that is up to 100 times more powerful than morphine so they can give it to patients when paramedics are unavailable.

As Ambulance Victoria tries to expand its use of lesser trained first-aiders and improve its response times, 146 volunteers in 28 locations have been taught to administer Fentanyl where it is not economically feasible to maintain professional ambulance branches.

The move has been criticised by the paramedics' union and the Victorian Emergency Physicians Association as dangerous, with the latter saying it could be a slippery slope that leads to volunteers taking on more advanced medical work they are not qualified for.

Fairfax Media has also learnt that the volunteers, who receive eight hours' training to administer the highly addictive opiate to patients in extreme pain, do not carry Naloxone, the antidote to bring people back from an accidental overdose.

Association president Dr Alan Whitehead said administering the potent drug without an antidote was risky because calculating the dose could be difficult and overdoses could cause brain damage and, in rare cases, death. ''Even fully qualified ambulance paramedics don't get the dose right from time to time so that is more likely to happen with people who have less … experience,'' he said.

The controversial program was signed off on by Ambulance Victoria's medical advisory committee in June 2012, three months before management discovered staff had been systematically siphoning off Fentanyl from ampoules and replacing it with water to avoid detection. The theft, which raised concerns patients had been given water for pain relief, resulted in arrests and prompted security upgrades for the Schedule 8 drug.

But the general manager of regional services for Ambulance Victoria, Tony Walker, said the scheme was safe.

He said 146 of the service's 1000 volunteers, who initially received at least 96 hours' training to become ''first responders'', had been given an additional eight hours' training to administer Fentanyl. Training had been provided only to volunteers in isolated areas such as Rainbow, Penshurst, Cann River and Mitta Mitta.

Ambulance Victoria medical adviser Professor Stephen Bernard said volunteers had administered Fentanyl 54 times in the past year without any adverse incidents.

But the secretary of the paramedics' union, Steve McGhie, said given paramedics had to complete a three-year degree and be monitored on the job for 12 months before they could administer Fentanyl alone, the plan was dangerous.