A toxicology report released Monday showed levels of cyanide in Giuseppe De Vito's blood and in his stomach.
De Vito died at the Donnacona Institution, a maximum-security federal facility near Quebec City. He was found unconscious in his cell last July 8.
The convicted mobster was serving a 15-year sentence for drug trafficking.
He was the husband of Adele Sorella, a Quebec woman who was found guilty this past June in the slayings of the couple's two children.
Those March 2009 deaths occurred while De Vito was on the lam from police.
When De Vito was arrested in 2010, he'd drastically changed his appearance.
He was suspected of being one of those involved in a play for leadership of the Mafia a few years ago in Montreal.