Canadian pharmacies won't fill a prescription written by a US doctor in my personal experience. State to state is fine but out of the country doesn't work because of the varying things like the DEA # required in the US for doctors to write prescriptions.
I tried Sativex back in 2007. Since it's not a scheduled drug, you can go to any walk in clinic and get it prescribed easily. This is what I did while I was traveling. The first clinic I walked into was plastered in "WE DO NOT UNDER ANY CIRCUMSTANCES PRESCRIBE PAIN MEDICATIONS, SO DO NOT ASK!!!!" on every single wall in the waiting room, on the entrance doors etc. I told the doctor that I had MS and that I wanted to try Sativex for the pain. They were like "it's not a scheduled drug so it shouldn't be a problem to prescribe it." The doctor asked his boss and I walked out with a script for it. The pharmacy that I went to had to order it in since they didn't have it in stock but they had it delivered the next day.
Now this is based purely on my experience in 2007, before Sativex was even in trials in the USA. The one thing that I want to make clear is that at the time, it was still technically "illegal" to possess in the US in theory because it's pure THC (with gross mint added in!) but I never had any issues. I don't know if it's still in trial phases or if it's scheduled in the US but like any prescriptions, always keep it in the prescription bottle (mine didn't have a prescription sticker on it, only on the pill bottle it came inside of) and I don't think you'll have an issue with it since it is legal in Canada, which is a lot different than going to The Netherlands and coming home with a pound of weed and being like "but it's legal where I bought it!!"
Good luck with it! I personally thought it was a great alternative to smoking because I didn't have to deal with feeling "high", which I cannot stand, and it did help but as my MS has progressed, I have found that the way my body reacts to it is different every time. It doesn't matter if I smoked the same medicinal marijuana from the exact same pipe on Monday as I did Tuesday but on Monday while it would just make the pain feel like warmth, on Tuesday it may turn that pain into overdrive as my sensitivity was cranked to the max. It did work for a while but it was too inconsistent for me. Like I said, good luck!