Doctors who prescribe amitriptyline for pain sometimes explain to the patient that amitriptyline is an antidepressant drug that can help with pain, but often these doctors deliberately deceive the patients into believing that amitriptyline actually is a painkiller and do not mention that it is an antidepressant, and that they are prescribing it because they have decided the patient is 'a depressive', not a person in pain. Doctors often confuse depression and pain, unfortunately.
Now 'dry mouth'/lack of saliva obviously makes dental decay more likely, especially of the lower teeth, since normally many of these are bathed in saliva most of the time, and this inbuilt natural 'mouthwash', therefore, protects the teeth to some extent from food residue clinging to them. Furthermore, saliva is a buffer solution which chemically protects teeth from acid attack. So existing mouth pain is likely to be augmented by the development of dental caries, making matters worse, not better.