I've never done it. The product tends to irritate the scalp if you have any easily irritated spots already so I imagine that irritation would get worse the longer it was left on, indicating you need to stop the process promptly. Rather than damaging hair, I think the problem would be with the hair follicles and this might lead to hair loss. I have used the product for many years, only straightening roots every 12 weeks or so - I've never experienced any hair damage or loss.