Some suggestions from a student applying for UBC Medical School from Mathematics
Informal Advice
 
This advice was kindly provided by a student applying to UBC Medical School in the 2011 cycle. While very thoughfully prepared by the student, you should of course read carefully the Medical School information for the school you are applying to. A number of students have gone from Math degrees to Medical School.
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The MCAT is unimportant at UBC, which is very 'strange' when compared to most schools. UBC generally uses the MCAT as a baseline for entry, and it is only used in 'tie-breaking' equally qualified applicants. For most schools, it is of very high priority.

As for studying, the MCAT was quite the experience for me. Having 0 high school biology, and no biochemistry hurt, it showed on my scores. I studied by myself for one month while working full time, and pulled off an impressive score. As for specialized training, that is something you certainly need. It requires critical analysis more than factual knowledge, but without a baseline you can't succeed. Speed reading, comprehension, and reasoning are emphasized more than anything, and so that is what I worked on the most.

For many schools, volunteering and leadership are important. You need to have diverse experience more than hours, and location is irrelevant (they don't rank hospital volunteering over cupcake sales), but it is best to at least TRY medical volunteering, if only to know what you are getting into. My training in this area was mostly from volunteering at local seniors' homes, specifically in dementia ward. UBC values volunteering equally to grades, while most schools adjust grades as being more important.

Marks are probably the most important thing of all. An 80+ average is a must, anything 85+ and you are stellar. Most successful applicants have these qualifications.

Finally, the interview trumps all. Once you have the grades, the volunteering, the MCAT, you will either be accepted for interview, or not.  If you do get one, all else becomes almost irrelevant. The interview is a 3 hour intense, rapid fire set of scenarios. This requires you to not only have some general understanding of medical ethics but to be a genuinely confident, humble, and well spoken person.

After that, its all chance. I'm sure there are always other things considered... do you share a surname with a wing of the hospital, are your parents friends with the medical council members. Of course, you can't control this, so its best not to worry.

Hopefully this covers everything anyone could care to know. One final thing to note for 'Math' majors is.. major doesnt matter, medical schools want people from diverse backgrounds, not biology wana-be's who consider no other options. Don't ever think that graduate schools wont consider you for pursuing an undergraduate degree in Mathematics, assuming you have equal marks and volunteerism.