It depends on the specialty as some have different training times in Canada versus the US. If the training in the US for the specialty is equal to or greater than Canada, then I believe it is not hard to come back. I believe if the training time is shorter in the US (e.g. ER is 3 years in the US and 5 in Canada), you have to get some kind of additional training or supervised practice first, although I'm not entirely sure of the specifics.