In one of the largest murder cases in Ohio’s history, a doctor was charged on Wednesday with killing 25 people over four years by prescribing fatal doses of fentanyl, a powerful opioid, to critical-care patients at hospitals in and around Columbus.

The doctor, William Husel, 43, pleaded not guilty at his initial court appearance, and a magistrate set his bond at $1 million. The authorities have not described a motive in the case.

A lawyer for Dr. Husel said that his client had tried to provide comfort to dying people, not to kill them. “I can assure you there was never any attempt to euthanize anyone by Dr. Husel,” the lawyer, Richard Blake, said on Wednesday as his client turned himself in to the Columbus police.

Dr. Husel, who had worked for the Mount Carmel Health System since 2013, was fired in December; the state medical board later suspended his license to practice. A pharmacist had alerted hospital authorities that Dr. Husel was prescribing high doses of fentanyl to patients, in some cases overriding the hospital’s protocols for the drug.