An elderly patient taking 5+ medications (including propranolol, citalopram, and hydrochlorothiazide) and smoking two packs of cigarettes a day has no expressible saliva on palpation of the Stensen's and Wharton's ducts. What is the most likely cause?

While heavy smoking has its own oral health effects, the patient is on multiple medications independently well known to cause significant xerostomia (a beta-blocker, an SSRI, and a thiazide diuretic are all classic xerostomic drug classes) — polypharmacy with several xerostomia-causing medications is a far more potent and well-established cause of a near-complete absence of expressible saliva than tobacco use alone.