Nicolas Bonnemaison, 54, was found on Saturday morning slumped in a vehicle in the village of Tosse in southwestern France after apparently taking medication.
He was transported by helicopter to a hospital in nearby Bordeaux, where one source said his condition was stabilising.
On October 24, an appeals court in the western city of Angers handed Bonnemaison a two-year suspended sentence for killing a patient, an 86-year-old woman.
But it backed a lower court ruling that had acquitted him last year of the poisoning deaths of six other terminally ill patients.
In an emotionally charged trial that gripped a country where euthanasia is illegal, Bonnemaison was accused of “poisoning particularly vulnerable people” — five women and two men who died between March 2010 and July 2011 soon after being admitted to the hospital in the southwestern city of Bayonne where he worked.
He had told the appeals court that he administered the injections in order to “relieve but not to kill” patients who were suffering.
The appeals court ruling had come just a day after France's highest administrative court authorised ending the life of a quadriplegic in a vegetative state, a landmark decision that divided his family and was immediately blocked by the European Court of Human Rights pending a review of the case.
Euthanasia remains illegal in France despite recent efforts to ease legislation dealing with the terminally ill — a campaign promise by President Francois Hollande. In March, lawmakers voted overwhelmingly in favour of a law allowing medics to place terminally ill patients in a deep sleep until they die.