Dandruff is the common name for a mild form of seborrheic dermatitis of unknown cause. It is a natural and harmless scalp condition in which the shedding of dead skin cells occurs at an unusually fast rate. Because of the oily skin often associated with this condition, these cells clump together and flake off as dandruff.
Dandruff is a condition. Shampoo that aims to treat it is called 'Dandruff shampoo' or 'shampoo that treats/prevents dandruff'.
While I've seen shampoo called "...for healthy/lovely hair" or "for dry/greasy hair" ( in which the fors have different meanings) I've never seen a shampoo that says it's *"for dandruffy* hair".
*PS Of course, the makers/promoters of cosmetics and heath products are quite happy about making up new words; if you're trying to make people feel there's something worth paying for, it's tempting to invent a word for it. There's a kind of mascara that claims to be volumizing. So I'm not at all surprised if some dictionaries recognize 'dandruffy'; it's just that this native speaker has never met it.
Last edited by BobK; 15-Dec-2008 at 15:32.
Reason: Added PS