How do I treat dry mouth caused by medication?

Dry mouth meds. You should talk to your doctor, have him change the meds if your dryness is so bad you can not tolerate it.
Not a simple problem. Confer with your physician about the possibility of substituting another medication (which hopefully won't have that side effect), but that isn't always possible. Sucking on a lemon drop or other sour candy can promote salivation, but it must be sugar-free, since dry mouth puts one at great risk for cavities. Biotene makes a line of products which have proven helpful at combating dryness.