A drop (as in a dropped class during the drop-add period) is not the same thing as W (withdrawal). The former is never reported; the latter always is.
Both W and P/F grades in science and pre-req coursework require explanations. It’s something that will be asked about should the applicants be fortunate enough to get an medical school interview.
A good explanation can mitigate the negative perception of a W. (And by good explanation I mean something like, a member of my immediate family died and I was so distraught that I knew I couldn’t do my best. Or I experienced a significant illness and missed too much classwork to make up. Or I realized that enrolling in 22 credits with 3 lab classes while working 20 hours/week to pay my tuition was a mistake and I couldn’t dedicate adequate time to all my coursework to be successful.)
Taking a class P/F, then taking it again for a grade will require an equally good explanation to mitigate the negative perception that act generates. Saying I took the class P/F to see how hard it is or to see if I could do well before taking it for a grade isn’t going get any sympathy from adcomms. About the only explanation I could see an adcomm having any sympathy with would be: I was history major fulfilling a graduation distribution requirement and never thought I’d be applying to med school so I took the class P/F, but now 10 years later I changed my mind and realized I needed to have a grade for bio/chem/physics/math to apply to med school so I needed to retake the class.
Like @Jugulator20 said, medical school is expensive (both for the student and for the school). They want to enroll students who will be successful, students who have proven they can master large quantities of difficult science material quickly during a single exposure to the material.