If you become a citizen or a permanent resident (landed immigrant), you can apply just like any Canadian. If you apply as an American without immigrating to Canada, you will be treated as a foreign student. I believe only three schools accept foreign students- McMaster, University of Toronto, and McGill; however, it is quite difficult (and expensive) for a foreign student to be admitted to any of these. All medical schools are public institutions that have a mandate to provide physicians for a certain region - a foreign student is simply less likely to practice in Canada. In addition, Canadian medical schools place a greater emphasis on academic achievement than most American universities. This results in higher GPA requirements than most American schools. Extracurricular activities are mostly an afterthought in the admission process here.
You will be eligible for provincial loans if you are a permanent resident/citizen residing in that province.
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