CBC News reported on Thursday that coroner Renee Roussel told Radio-Canada the concentration of the chemical DEET, a primary ingredient in insect repellent, in the sisters' systems wasn't sufficient to be fatal.
The claim contradicts the conclusion of Thai authorities who performed post-mortems on the bodies of Noemi Belanger, 25, and Audrey Belanger, 20, shortly after the sisters were found on June 15 by hotel staff.
A pathologist determined the women likely ingested DEET in a euphoria-inducing cocktail that is popular among youth in Thailand.
The sisters were students at Laval University in Quebec City. They worked at their father's grocery store in their home town of Pohenegamook, about 200 kilometres northeast of Quebec City.
They had arrived on Phi Phi Island and were last seen partying with two Brazilian friends in the early morning of June 13.
Investigators said there were no signs of foul play in their hotel room, but there was evidence that the women may have suffered some kind of toxic reaction. Officials speculated the women probably died of food poisoning.
However, CBC News reported that Dr Rene Blais of Quebec's poison control centre said the DEET concentration reported by the Thai pathologist does not meet the concentration level that that would be toxic, "let alone a concentration that would be fatal".
It's still unclear what caused their deaths if it wasn't DEET poisoning.
Secondary autopsies were conducted in Montreal, but the results haven't been released.
Thai investigators haven't closed the case. They submitted their investigation report to the Canadian embassy in Thailand without making the findings public.
Pol Col Boontawee Toraksa, the then deputy commander of the Krabi provincial police, said on June 28 that Ramathibodi Hospital had sent an initial result of its autopsy to him. The autopsy found no traces of drugs in the sisters' bodies.