Hi! is constantly masturbating cause any harm? something about parasympathetic nerves?

Masturbation: Nope. Unless you count the potential of abrasions from so much friction. Masturbation is a normal and common part of the human experience. There are no negative health consequences to masturbation.

Constantly?: If you're obsessed with erotica and are missing out on the rest of life and other things you need to be doing, tell your physician about it -- he's heard it before -- and a bit of medication and/or learning some mind tricks should fix you up. Entertaining yourself up to several times a day is otherwise risk-free & actually prepares you for marriage / adult life. Think about that one special person.