Here in Quebec, you need to go to a clinic and get your US doctors prescription transferred to a Canadian prescription, cost about $50.00 CDN, (about $38.00 USD), if you don't have RAMQ, (Quebec provincial health care card).
However, I have had friends from the US go to the pharmacy with their empty prescription bottles and say that they "forgot" to fill it before leaving the US. The pharmacist gave them a one month supply each time. Not sure if this would work in other provinces.
I hope this helps.
2004 Damon Challenger 348W, 2011 Ford Taurus SEL
US Army veteran, USAF disabled veteran, (yes, I served in both branches).