An appeals court handed Nicolas Bonnemaison a suspended two-year sentence in 2015 for deliberately poisoning an 86-year-old woman shortly after she was admitted to a hospital in the southwestern city of Bayonne, where he worked.
He was also suspected of poisoning six other patients at the hospital in 2010 and 2011, but was acquitted in those cases.
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Yet Bonnemaison argued that France's Medical Council had disbarred him in 2012 over the alleged ethical breaches, even before his first trial in 2014, which saw him acquitted on all counts.
In his filing at the European court, he argued that the council's disciplinary proceedings were not impartial and violated the presumption of innocence.
In its ruling, the court rejected all of Bonnemaison's claims, saying it had found no evidence to suggest bias by the disciplinary council.
Euthanasia is illegal in France despite several efforts in recent years to allow doctors to consider it for the terminally ill.
Bonnemaison's emotionally-charged trials gripped the country at the time, with dozens of right-to-die advocates supporting him at the hearings.
The legal troubles also took a toll on Bonnemaison, who attempted to commit suicide a few days after his 2015 conviction on appeal.
The right-to-die debate has been revived recently in France with the case of Vincent Lambert, 42, who has been kept alive in a vegetative state for years following a car accident in 2008.
Doctors have been urging that life support be removed for Lambert, who was left a tetraplegic with extensive brain damage after the accident.
Despite support for the move by his wife and six of his eight siblings, as well as French courts, Lambert's parents contest the decision, saying their son's condition could improve if he gets better treatment.