Canadian pharmaceutical billionaires Barry and Honey Sherman were murdered in a targeted killing, according to police in Toronto.

However, no one has been charged with their deaths.

Police disclosed that they were treating the case as murder six weeks after the Shermans were found dead in their Toronto home.

Toronto police detective Susan Gomes said she was "not going to discuss suspects" but believed the Shermans were targeted.

In a statement, the Sherman family said the double murder designation "was anticipated" by the family.

The family had long maintained that an initial investigation into the case as a potential murder-suicide was faulty. They had hired a private investigator and conducted independent post mortems.

In the days immediately after the Shermans were found dead, multiple news organisations quoted police sources as saying investigators were operating a working theory that the deaths were a murder-suicide.

"I don't know where that came from," Ms Gomes said, adding that police gave equal weight to three possibilities: double suicide, murder-suicide and double murder, before deciding the evidence supported the latter.

The Shermans' deaths stunned Canada's business, political and philanthropic communities, drawing public condolences from people including Canada's prime minister, Israel's consul general, and Toronto's mayor.

Barry Sherman founded Apotex Inc and built it into a pharmaceutical giant.

He and Honey Sherman became known for their philanthropy, donating to hospitals, universities and Jewish organisations.