I have approximately 7 W's on my transcript and two incompletes turned NCs(freshman course grade forgiveness turns "F"s into "NC"s), and I just recently got accepted, so W's (especially if you just have maybe one or two) are certainly not insurmountable. My case was a bit extreme, so I explained the circumstances in my personal statement, but in your case, if its brought up in interview, just have an explanation at hand. Certainly, some explanations will hold more merit than others, but somehow, I don't think one or two Ws will cause them to think "this person is a bad candidate for medical school". A long history of dropping a course or two each semester, on the other hand, would take a lot more "explaining away".