A DIN is not the only answer. A medication must also be included in the provincial formularies (lists). Even if it is an approved drug, if there is another drug that works better, is cheaper, or more politically correct, it will be on the formulary and yours might not, so you may still be out of luck.
For example, Ontario's Trillium drug plan does not cover the triptans I take for migraines (any triptans from any manufacturer), and also does not cover the painkillers that I need if I can't get the triptans. So I'm pooched either way. Most government organizations like workers comp follow the approved provincial formulary. Most private insurance companies have their own lists, and they don't cover everything either.