causes for dry mouth?

My tongue has been feeling very irritated for about 6 months.  It even feels like it's flaking off in tiny pieces.  Could this be dry mouth?  Sometimes the roof of my mouth is also sore, as well as under my tongue.  This condition causes me to "chew" constantly, due to the sensation that something is on my tongue, which causes more irritation.  

Sometimes I wake up at night and my tongue is almost glued to the roof of my mouth, its so dry.

I went to my dentist for help and he was baffled - recommended changing tootpaste to a plain one, and using antiseptic mouthwash, which has not helped at all.  

Any clues as to what might be causing this condition?  (I have never have been a smoker.) The only thing that I can think that I've changed is that I've been adding 800 IU's of Vit. D.

[Mother Margaret]

Maybe it is a sign that you have been exposed to too much of some kind of chemical:  mouth ulcers is a sign, maybe dry mouth is also.  http://www.valdezlink.com/pages/howcomplain.htm

What has changed in your life since this started?  

[fullmergavin]

The only medical condition I could think of in this situation is Sjogren's Syndrome. There are quite a few vitamin deficiencies and imbalances that have been traced to these types of ordeals, too. In the case of the mouth, B6/B12/Folic Acid, Biotin, and Riboflavin would tend to be the areas of focus. Which one, or how many, who knows.