I've used dandruff shampoo for years to help control my dandruff and my hair has not been harmed from the use of it.
So in my opinion dandruff shampoo is not bad for your hair and will actually help improve your hairs health by removing the dandruff.
The dandruff itself could do more harm to your hair than the dandruff shampoo could ever do.
Dandruff shampoo is formulated to help remove and control your dandruff so that your hair stays healthy.
You can use dandruff shampoo as much as you want and it won't do your hair or scalp any harm.
But I use the dandruff shampoo twice per month and the other weeks I wash my hair using the regular shampoo.
I haven't had as much dandruff after using the dandruff shampoo regularly and my hair is perfectly fine and healthy.