I think it is caused by over production of sebum (don't know if that's the correct word?) in the scalp. I use a leave-in conditioning lotion that restores balance to the scalp, treats dandruff, and calms the scalp. I have tried products with "pyrithione zinc", but that wasn't what worked for me. You can try out a few different treatments to find one that works for you. The product I use is Scandinavian, so you probably can't get that, but you can read about it on
Cutrin if you'd like to try to find something similar. On the homepage click english in the corner then Products > Series > Bio+ > Dandruff > Active Care.