Holmes uses addictive drugs, which he turns to especially when lacking stimulating cases. Holmes is a habitual cocaine user, which he injects in a 7 percent solution, often using a special syringe that he keeps in a leather case. Holmes is also an occasional user of morphine, but expressed strong disapproval when he visited an opium den. All three were legal in late-19th-century England.

Dr Watson reflects Victorian medical orthodoxy by having no medical objection to Holmes' drug use. Morally, however, he disapproves of his friend's drug use, describing it as the detective's "only vice," and expressing concern over the effect it may have on Holmes' mental health and superior intellect.[18][19] In later stories, Watson claims to have "weaned" Holmes off drugs. Even so, according to his doctor friend, Holmes remains an addict whose habit is "not dead, but merely sleeping."[20]
-I took it from google,sorry..

I'm STILL freaked because Holmes is my god &,I mean, think about it, god does cocaine?! WTF?!

Every part of character is imporant and gives us a view, if it's a story, movie or TV show or anything at all. Even if in real life that would be dusturbing, here it makes character even more interesing.

it's he's personality.