It burns and weakens fabric, the same goes for your hair. If you want it done, I suggest by someone with decent experience. If your doing it at home, look up some advice and how to's. Rinse thoroughly!
Not any more than bleaching your hair normally would. BUT you have to rinse rinse rinse and rinse even more when you use chemicals like that on dreadies. The chemicals can get trapped inside, eating away at the center of your loc, killing it. It's find to bleach and dye, just be really super extra careful.