Nicolas Bonnemaison faces life in prison if found guilty of "poisoning particularly vulnerable people" -- five women and two men who died between March 2010 and July 2011 soon after being admitted to a hospital in the southwestern city of Bayonne where he worked.
The case came to light in August 2011 after the emergency room doctor was denounced by nurses and a carer who had noticed suspect behaviour and comments by Bonnemaison, who once reportedly said: "I'll take care of it personally, this has lasted far too long."
Euthanasia is illegal in France but President Francois Hollande promised during his 2012 campaign to look into legalising the hugely divisive practice.
A 2005 law in France has already legalised passive euthanasia, where a person causes death by withholding or withdrawing treatment that is necessary to maintain life.
Late last year, a panel set up by Hollande also recommended legalising assisted suicide, which allows a doctor to provide a patient with all the necessary lethal substances to end their life, but lets them carry out the final act.
Bonnemaison is accused of having given drugs to the patients without asking other doctors' advice. He reportedly used Hypnovel, a powerful sedative, and Norcuron, a muscle relaxant -- both used for general anaesthesia.
"I'm thinking of the patients today... You experience powerful things with terminally-ill patients, things that leave their mark on doctors. They leave their mark on the human being too, beyond our job as doctors," the 53-year-old told the court.
"It's important that the family be informed so that they can understand what is going on," he said, hoping he had been "up to the challenge" before adding: "I did what I could."
The trial of Bonnemaison, who has been struck off the medical register, has stirred up considerable emotion, including among the victims' families.
Patricia Dhooge, the wife of Fernand -- one of the victims -- said she would testify for the doctor.
"I'm having a lot of trouble understanding this trial. I want to tell the court 'we were there, we agreed'," she said.
"I pleaded (with Bonnemaison) to stop his suffering, morphine was of no use anymore... Fernand knew what he wanted. It wasn't brutal, we were there with his three children, we were able to kiss him. I want Dr Bonnemaison to be acquitted."
A petition in favour of the defendant has also garnered 60,000 signatures.