Started pre-med, finished 3 years including calc, stats, gen-chem, o-chem, physics, and bio, all with lab, all with grades good enough to go on to med school. Got to upper-division sciences and found it utterly boring and worthless to me. Switched to public policy, graduated, now in grad school for public health policy.
Something to keep in mind is that you can go to med school from any major or discipline as long as you have completed the requisite requirements (essentially what I listed above, sometimes also with biochem). You're also expected to be an ideal student in your other classes, do research, and volunteer work and kick ass on the MCAT (which is a very difficult test). It's a lot of work. Look into it in a fair amount of detail and be fully committed to it if that's what you know you want to do. Trying it half-way will just end up in a let-down.
Doesn't mean you need to be a loser in school, my one friend was one of the biggest partiers in my fraternity but spent about every other waking moment working towards his goal so he had the opportunity to get stupid drunk rather often. Now he's in his second year at a UC med program and will make a great doctor.