Frederic Pechier, 47, now stands charged in 24 cases, nine of which resulted in death, after working as an anaesthesiologist at two private clinics in the eastern French city of Besancon, where seven patients, aged 37 to 53, went into cardiac arrest from 2008 to 2017.
If convicted, Pechier could face a life sentence.
A source close to the investigation said he would go before a judge later Thursday to determine whether he should be held in custody, as demanded by prosecutors.
Pechier's lawyer Jean-Yves Le Borgne said the investigation had proved nothing.
"There is a possibility that Dr. Pechier was the author of these poisonings but this hypothesis is nothing but a hypothesis and this long inquiry over two years has shown nothing ... to the extent that in this instance, the presumption of innocence must be stressed," Le Borgne said.
Pechier he was brought in for questioning earlier this week over 66 more suspicious cases of cardiac arrest during operations on patients otherwise considered at low risk of such incidents.
"Seventeen cases have been retained" involving patients aged 4 to 80, of whom seven died after doctors were unable to revive them, local prosecutor Etienne Manteaux told a press conference.
Pechier was "the common denominator" in the new cases, which occurred at a time when he was in open conflict with fellow anaesthesiologists at the Saint-Vincent clinic in Besancon, Manteaux said.
"He was most often found close to the operating bloc" when the cases occurred, and made quick diagnoses of the problem and the action to take, "even when nothing allowed anyone to suspect an overdose of potassium or local anesthesia," he said.
Prosecutors have alleged he may have tampered with his colleagues' anaesthesia pouches to create operating room emergencies where he could then intervene to show off his supposed talents.
Pechier's lawyers have denied the claims and in November they accused police of altering declarations he made during his initial questioning.
During questioning this week, Manteaux said Pechier acknowledged that criminal acts had taken place at Saint-Vincent but that "he was not responsible for these poisonings."