A Halifax law firm has filed a proposed class-action lawsuit against Health Canada, accusing the federal department of violating the privacy of medical marijuana users.

Marijuana Medical Access Program

These letters from Health Canada were sent to about 40,000 medical marijuana users across Canada. (CBC)

Last week, the federal government began sending letters to all those in the medical marijuana access program, explaining upcoming changes that roll out April 1.

The problem is the return address on the outside of the envelope was clearly marked with 'Marijuana Medical Access Program,' and included the user's name and address.

That has some medical marijuana users crying foul, saying Health Canada has jeopardized their privacy, even their safety and job security.

One Nova Scotia man who is allowed to grow his own marijuana said many of his friends and family don't even know he's part of the program.

He's one of 40,000 Canadians across the country who can legally possess medical marijuana. Now, he's not sure who has learned he uses the drug.

"It said medical marijuana access program, right on the letter that was sticking right out of the mailbox for anyone who walked up on my step to see," he said. "Could have been a neighbourhood kid knock on my door, trying to sell tickets to a raffle, that seen that sticking out of there."

The man doesn't want to be identified, in part to keep his family safe. He's also worried about losing his job, where a security clearance is required.

Health Canada has apologized for the gaffe, calling it an administrative error. It is refusing to comment on the legal action.

McInnes Cooper lawyer David Fraser is representing some medical marijuana users and has filed a lawsuit that seeks class-action status in the Federal Court of Canada.

Fraser cautions it could take years to work its way through the courts. The action alleges the privacy of tens of thousands of Canadians was violated.

"Individuals are experiencing significant anxiety about their own security, the safety of their families," Fraser said in an interview. "In many cases, these are people who have small children in their homes. It also includes the elderly."