Employers in Canada have a duty to accommodate employees with medical disabilities that are using prescribed medical cannabis as a treatment.
I should say, at the outset, that I am a Human Resources specialist, and my perspective does not constitute legal advice of any kind.
Human Rights legislation in Canada requires employers to accommodate individuals with a mental or physical disability (amongst other protected grounds). Under this legislation an employer cannot discriminate against an employee or treat them differently because of their disability.
The duty to accommodate means the employers must make every reasonable effort to accommodate an employee who falls under a protected ground (i.e. disability) to the point of undue hardship. This includes accommodating the treatment options that follow the disability. [Read more at Civilized]