It's obviously not a 'basic' science though and other applied sciences that are uncontroversially included in STEM get their own letter (engineering and technology).
People differ in whether they think medicine and the other applied sciences fall under the 'science' umbrella or not. Try searching around and you'll see a lot of mixed opinions on the subject.
I guess you could argue that Tech and Eng getting their own letter means they don't fit under the "S" in STEM, so STEM might not include the non-enumerated applied sciences. (Nor fields of "applied technology", like trucking.)
People differ in whether they think medicine and the other applied sciences fall under the 'science' umbrella or not. Try searching around and you'll see a lot of mixed opinions on the subject.