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.
Google Sjogren's syndrome. It is usually misdiagnoised or not diagnosed at all...