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Safe is a relative term, but I'd say they are. Depending on the position, shape, and color of your teeth, they will need to be prepared ('shaved down') a certain extent so as to allow room for the veneer to be bonded. If your teeth are relatively straight and not too discolored, then very little tooth structure will need to be removed (0.3mm-1.0mm.) Basically, the more mal-aligned your teeth are or the more discolored they are, the more prepping has to be done. This could possibly lead to a pulpitis and the need for a root canal. Usually it's good to have ortho to pull some teeth into a better position first before having veneers made.

Source(s):
I'm a DDS