A critical care doctor in the US state of Ohio has been charged with murdering 25 patients, by allegedly ordering fatal doses of pain medication.

William Husel (43), formerly a physician at Mount Carmel Health in Columbus, pleaded not guilty to all counts at a bail hearing on Wednesday after turning himself in to police.

Each murder count carries a sentence of 15 years to life.

The deaths are alleged to have taken place between February 2015 and November 2018 and involved the synthetic opioid fentanyl, said to be 80 to 100 times stronger than morphine.

The investigation into Dr Husel began after a lawyer representing Mount Carmel contacted prosecutors after concerns were raised about his actions by another member of staff.

Ed Lamb, president and CEO of Mount Carmel Health System, said: "Following the discovery of the actions of Dr Husel, we notified appropriate authorities, including law enforcement.

This file photo shows the main entrance to Mount Carmel West Hospital in Columbus, Ohio | Image: Andrew Welsh Huggins/AP/Press Association Images

"We have shared information with them and will continue to fully cooperate throughout their investigation.

"Mount Carmel has made and will continue to implement meaningful changes throughout our system to ensure events like these never happen again."

At a news conference on Wednesday Franklin County Prosecutor Ron O'Brien said the inquiry had been focused on cases in which 500 micrograms or more of fentanyl had been prescribed.

"The expert witnesses that we talked to said, to a certainty, that a dosage at that level could not support any legitimate medical purpose and a dosage at that level would cause someone's death," Mr O'Brien said.

Police Chief Tom Quinlan said that around December 7th 2018, detectives had received information that "numerous patients" had died while being treated by Husel while he was at Mount Carmel Health.

"This breach of a doctor's oath is vile and worthy of today's actions - and that begins the process of holding this killer criminally accountable," he said.

Main image: Former critical care doctor William Husel (centre) pleads not guilty to murder charges in the deaths of 25 hospital patients while appearing with defence attorney Richard Blake (right) in Franklin County Court in Columbus, Ohio | Image: Kantele Franko/AP/Press Association Images