Google Sjogren's syndrome. It is usually misdiagnoised or not diagnosed at all...
Could be Sjogrens. They can do blood work and a salivary gland biopsy for a diagnosis. I've had chronic dry mouth and throat, plus other problems, for about 3 years and have seen numerous doctors and nobody can diagnosis this. It's horrible.
I had the salivary gland biopsy done and it wasn't too bad. It was done with a local in the doctor's office. That must have been 2 years ago now and the numb feeling was supposed to last 4-6 weeks. Well, my lip is still half numb.
It's miserable, I know!! Mine is so bad, it has caused anxiety attacks and I am just about confined to my home. Keep drinking fluids, esp water. Try the Biotine products and I get a gel from my dentist (called GC Dry Mouth gel) that I rub on my gums and tongue at night. I think there are other gels out there too. Try chewing sugarless gum. Now, I'm having dental problems due to all this.
Good luck.