I think that the marker or pen harm your skin is some way, maybe what it contains or something? I heard that it causes cancer as well, but who really knows. I have written on my hand since I was little and still, I'm pretty healthy :)
If the ink is toxic, then yes it is because it can poison you, but since law requires ink to be non-toxic, it won't do any harm, it will just fade away and absorb into the skin
You could be allergic to the ink in the pen/marker you're using. That's what happened to me and I had really bad hives for like a week.
dont know how true, but i was told it causes cancer?!
Ummm cuz it wasn't made to write on