Well, witch hazel has a certain amount of alcohol in it.
When you put alcohol on your face, it strips away the natural oils. This sends your skin into confusion, thinking it needs oil. Then it starts over-producing it, and your skin ends up oiler than before. Toners and astringents like witch hazel have a small percentage of alcohol (some more than others). Overuse of these can still cause problems.
If you put in on your scalp, it may do the same thing. If so, you could end up with a much oiler scalp, the culprit in dandruff.
Now, witch hazel has a small percentage, like 14%, but if you decide to use it, try not to overdo it.
HTH /images/graemlins/smile.gif