Rye Hunt's family believe the Australian backpacker may have unknowingly taken a highly potent hallucinogen before he disappeared in Brazil more than a fortnight ago.
Mr Hunt's uncle and girlfriend, who are in Rio searching for the missing 25-year-old, now believe he may have taken hallucinogenic drugs rather than MDMA as previously thought.
"Reports from those around Rye in the lead up to his disappearance (described) irregular and out-of-character behaviours, such as extreme paranoia, anxiety and fear," the family said in a statement on Tuesday.
"Based on these effects we believe Rye may have unknowingly consumed a different illicit substance while thinking he was consuming MDMA.
"We now believe the illicit substance to be a highly potent hallucinogen and psychedelic often referred to as NBOMe."
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Mr Hunt has been missing since May 21 when he left Galeao International Airport in a taxi after an argument with his travelling companion, Mitchell Sheppard, and booked into a Copacabana apartment.
It's believed the two Australian backpackers had earlier taken drugs at a party in Lapa.
Mr Hunt's partner, Bonnie Cuthbert, and uncle, Michael Wholohan, are in Rio de Janeiro working with local and Australian authorities to find the missing man.
They revealed on Monday that Brazilian authorities had called off a 72-hour sea search after a fishermen spotted a body floating off Rio's coast in clothes similar to those worn by Mr Hunt when he vanished.
Authorities had previously scoured Cotunduba Island after another fisherman reported seeing Mr Hunt there - scratched and asking for water - the day after he disappeared.
Mr Hunt's family on Tuesday appealed for police and locals in Rio to continue their efforts.
"(We) determinedly continue to search for Rye," they said.
"The local Brazilian community have been receptive ... and willing to talk about our search."
