A undergrad is much more significant that "prepping" for med school. It should be something you enjoy first of all.
With your logic, you should also be heavily considering a major in Biochemistry, Neuroscience, or Physiology, because they too have places in the MS1/MS2 curriculum. Also remember that you probably will forget almost everything you learn in undergrad, so you are better off doing something that actually interests you.
Don't worry about what lies ahead in med school, get through undergrad in an enjoyable manner first. Everyone will make it through pharmacology, and prior experience would only give you a slight edge (if any). However, if pharm is what floats your boat, by all means go for it.
