This is really late, but may benefit someone else... I have had a similar experience within Canada...
When I left behind regular prescriptions (i.e. on file with a pharmacy) I went to a new pharmacy (same company, but I don't think that that matters), got their contact information. Then called my regular pharmacy and had them transfer the prescription to the new store. It took about an hour.
If you have had it filled at a pharmacy in the US, my guess is that it would be fine.
If it has never been filled, but you don't have the original script, I suspect you may have a problem, unless the pharmacy can call the Doctor's office to verify and get an original.
I don't imagine the country matters, assuming it's not something that hasn't been approved in Canada.