According to the CRA website, a prescription is not needed.  The marijuana needs to have been purchased from Health Canada and/or a designated producer.  Here is a quote, and the link for the quote:

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/330-331/dtlxpns-eng.ht...

"Medical marijuana – the amount paid to Health Canada or a designated producer for a person authorized to possess or use the drug for medical purposes under the Marihuana Medical Access Regulations or exempt under section 56 of the Controlled Drugs and Substances Act."

Also, the following link is where you would also see that a prescription is not needed as per the accompanying screenshot from CRA:

http://www.cra-arc.gc.ca/medical/#mdcl_xpns