i can't see them expecting you to have a prescription from a canadian doctor, since you are not a legal resident of canada yet. if you have the drugs, i suspect a doctor prescribed them in your own country. if you are coming to canada to land as an immigrant, then i can't see them questioning 6 mo. of medication that is needed for legit medical concerns. since you are immigrating, i suspect you will probably have some sort of medical records from your doctor, or proof of an ongoing prescription? border agents are humans after all. they aren't going to take away someone's medications, unless there is suspicion of illegal intent, which i'm sure will be easily proven wrong when they look up the type of medication you are bringing in.