HELP! how damaging is bleached hair?

Okay so I have naturally really thick healthy black hair ( i literally have been told i have shampoo commercial hair by so many people). I want to color my hair a medium neutral brown but in order to do that they have to bleach my hair a couple of shades with a very strong lift. How much damage do you think it will cause to my hair. I really love my hair and I dont want it to be really damaged.