hi it might be due to increased secretion like  oral infection , pancreatitis , mouth ulcers , gastresophageal reflux disease, liver disease , excessive starch intake . or decreased secretion as in Infections such as tonsillitis, retropharyngeal and peritonsillar abscesses, epiglottitis and mumps.Problems with the jaw, e.g., fracture or dislocation, neurogenic problem. treating or avoiding the underlying cause. Mouthwash and tooth brushing may have drying effects