I have never heard of any topical hair removal product being able to affect the tattoo. These products target hair folicules, not the skin. I suppose if you have used it before, and have never noticed any skin discoloration, then it is safe. It is not like a skin bleaching product, and for that matter, as long as there is no break in the skin on the healed tattoo, there is no way for the NAIR or whatever to get to the tattoo, it would stay on the surface.

I would however stay away from laser hair removal obviously, as it can and will dammage your tattoo.

There is always the mechanical hair puller contraptions, and good ol fashioned shaving.

Although it is notheworthy that repeated shaving over time will blur your tattoo, as the area around each hair folicule will develope a callus, causing the tattoo to be slightly blury

(you wouldnt be able to notice most likely, but is interesting to know) it doesnt stop me from shaving where i have to