It does horrible things. I straightened mine since 7th grade, and now I am on 9th. My hair is fried. I hate it. And I regret it. I even use a Tresemme heat protractor spray. Now I can only afford to straighten my hair a few times a week. That's not even the sad part! My hair was already naturally straight, and straightening it just made it a huge frizz! 1 out of every 4 hairs I examine are split ends.. And I got a trim just a few weeks ago. DON'T DO IT!