They have become addicted to fumes in discarded canisters of gasoline and kerosene.
The huge creatures inhale deeply before digging a shallow hole and lying back as the effects kick in.
Russian snapper Igor Shpilenok, 52, spent seven months with the community of bears.
He said some of the addicted predators even stalked helicopters, waiting for take off and drops of fuel to leak onto the hard soil for them to hoover up.
He added: “In another case a helicopter brought a few barrels of gasoline. Workers of the nature reserve didn’t take them in time and a female bear named Suzemka – who is apparently fasciated by the smell of fuel – used the opportunity.
“She seems to be one of the addicts.”
The Kronotsky Nature Reserve, South Kamchatka, in the far east of Russia, is home to more than 700 brown bears.
They are considered to be the largest brown bears in the world, weighing up to 1,200lb.
The beautiful sanctuary covers 225,000 hectares of land and is listed as a UNESCO World Heritage site.
The climate ranges between extremely hot and cold – earning it the label: ‘Land of fire and ice’.