Ambulance Victoria is contacting 19 patients who may have been given sterile saline instead of fentanyl or morphine for pain relief.

The ambulance service discovered in December that someone had tampered with vials of the painkillers and replaced the contents with saline in the Hume region, which includes Wangaratta, Benalla, Mansfield and Wodonga.

Police were called in to investigate and Ambulance Victoria conducted an audit of all medications being used across the whole service.

Ambulance Victoria withdrew access to the drug in parts of north-eastern Victoria while the investigation was underway.

The audit identified 19 ambulance patients who may have been affected.

What is Fentanyl? A commonly used synthetic opioid analgesic

A commonly used synthetic opioid analgesic Usually used to treat chronic pain in slow-release patch form

Usually used to treat chronic pain in slow-release patch form Fentanyl is 100 times more powerful than morphine and stronger than oxycontin

Fentanyl is 100 times more powerful than morphine and stronger than oxycontin Only available in Australia with a doctor's prescription

Twelve of those people were administered the drugs by nasal spray and are not required to take any action.

The remaining seven people are being urged to contact their doctor or Ambulance Victoria.

Tests on the tampered vials indicated they were not contaminated but those people affected are urged to discuss the situation with a medical professional.

Ambulance Victoria chief executive Greg Sassella has apologised.

"I offer my sincere apologies to these patients for not having been able to provide the level of care as effectively as could have been expected," he said.

"My thoughts are also with the paramedics who have unwittingly administered saline to patients in the mistaken belief that they were providing the pain relief required."

In 2012, two paramedics were arrested after hundreds of vials of fentanyl went missing and were replaced by tap water.

At the time Ambulance Victoria said all of the drugs would be kept in tamper-proof packaging to prevent a repeat of the incident.

Mr Sasella denies the organisation has a systemic problem.

"This recent one is confined to Hume," he said.

"We've introduced a number of measures since previous episodes.

"A number of people have been charged from the previous episode so we think it's well in control."

Police say the investigation into the thefts is continuing and that a Donvale man appeared in court in December charged with 169 offences of product tampering relating to fentanyl at a number of locations.