The deaths of two Canadian sisters on Koh Phi Phi in June were caused by the insect repellent DEET in their drinks, according to Canadian media reports.
Noemi (left) and Audrey Belanger were found dead in their room on June 15.
Results of the autopsy at a Bangkok hospital have not been publicly released, but they were shown to reporters for Radio-Canada, the French-language news network of the Canadian Broadcasting Corp.
According to the report, 20-year-old Audrey Belanger and her sister Noemi, 25, had DEET (diethyl-meta-toluamide) in their bodies.
Though the chemical is a potentially neurotoxic mosquito repellent, it is sometimes used as an ingredient to add an extra kick to a euphoria-inducing cocktail that is popular among young people in Thailand.
The cocktail known locally as 4x100 contains cough syrup, cola, ground-up kratom leaves, which are a mild narcotic, and ice.
It is thought that an overdose of DEET was accidentally mixed into the young women's drinks.
Large plastic buckets filled with different drink ingredients that are sipped through a straw are popular with Phi Phi partiers, who carry the buckets from place to place.
The sisters from Pohenegamook, Quebec had just arrived on Phi Phi and were last seen partying with two Brazilian friends in the early morning of June 13.
Forty-eight hours later, when staff at their hotel hadn't seen them emerge from their room, a receptionist called police.
The sisters were found dead in their room, and it was speculated that poisoning, probably accidental, played a role. Police said they found no signs of foul play.
However, officers did find a lot of vomit in the room and there was blood on the young women's faces, leading investigators to say early on that the women may have suffered some kind of toxic reaction.
Another autopsy is supposed to take place in Montreal, but final results from that are expected to take months.
In 2009, two young tourists, one from the United States and the other from Norway, who were staying at a nearby guest house on Phi Phi also died under mysterious circumstances.
Their deaths remain unsolved, but there was speculation the women had been poisoned.