Péchier could face a life sentence if convicted.
The 47-year-old worked as an anaesthesiologist at two private clinics in Besançon. Between 2008 and 2017, seven patients, aged 37 to 52, went into cardiac arrest while under anaesthesia at the hands of Péchier.
Lawyer Jean-Yves Le Borgne said his client had been freed on conditional release, after prosecutors requested he be held in custody.
"There is a possibility that Dr Péchier was the author of these poisonings, but this is nothing but a hypothesis. This long inquiry over two years has shown nothing... The presumption of innocence must be stressed," Le Borgne said.
Péchier was brought in for questioning earlier this week on 66 new suspicious cases of cardiac arrest during operations on patients normally considered low risk.
Seventeen cases were retained involving patients aged 4 to 80, seven resulting in death, said local prosecutor Etienne Manteaux.
In these new cases, Péchier remains the common denominator, said Manteaux, adding that they also took place during a time when the doctor was in open conflict with other anaesthesiologists at the Saint-Vincent clinic in Besançon.
Prosecutors have alleged that he may have tampered with his colleagues’ anaesthesia pouches to create operating room emergencies where he could then intervene to display his skills.
But lawyer defending the doctor deny such claims and back in November they accused police of altering declarations he had made during his initial questioning.
This week during another questioning, Manteaux noted that Péchier acknowledged that criminal acts had in fact taken place at the Saint-Vincent clinic, but that he “was not responsible for these poisonings”.