Medical Issues: Your Right to Privacy - Occupational Health & Safety Fact Sheet
If the status of your disability is in doubt; especially if there is time loss, the company has a right to require from you a medical certificate stating 1) If you are fit or unfit, 2) If you are unfit, the nature of your limitations (e.g. no heavy lifting, no stair climbing etc.), and 3) How long your condition is likely to continue. The choice of physician is up to you.