What exactly are the dangers of writing on yourself, with sharpie? or pen?
Depending on the brand, some brands' ink are toxic to your body. Which is why some markers are labeled 'non-toxic'.
However I don't think it should make much of a difference unless you drew on your entire arm with it. And I suppose for really sensitive people, they can start getting a reaction on their skin.
I've been doing it for years... so far so good, I dont think there's anything ever been proven...