PICKERING, Ont. - Police in southern Ontario have seized 42 kilograms of the deadly opioid carfentanil, saying it's believed to be the largest seizure of the drug in the country.

Carfentanil is a synthetic opioid similar to fentanyl but 100 times more potent, and an amount weighing less than a grain of salt can kill someone.

Durham regional police say they seized 53 kilograms of unknown substances from a Pickering, Ont., home on Sept. 20 and Health Canada has confirmed 42 kilograms of that was carfentanil.

They say a 33-year-old Oshawa, Ont., man is charged with possession of carfentanil for the purpose of trafficking.

Police also seized 33 guns and other prohibited devices in their search of the home.

The man was charged last month with 337 weapons-related charges, including careless storage of a firearm, unauthorized possession of a firearm and possession of a restricted or prohibited firearm.