Frédéric Péchier, an anaesthesiologist, has already been investigated for seven other cases of poisoning, in total leading to nine deaths.
Prosecutors allege he deliberately tampered with his colleagues' anaesthesia pouches to create an emergency and show off his talents.
Mr Péchier denies all allegations, and faces a life sentence if found guilty.
His lawyer, Jean-Yves Le Borgne, told AFP news agency that the investigation proved nothing.
