It is most commonly due to impaired swallowing due to autonomic dysfunction. Some medications cause sialorrhea by their effects on the cholinergic system.

Any neurodegenerative disease with autonomic dysfunction (e.g., Parkinson disease) can cause sialorrhea. Severe impairment in swallowing due to upper gastrointestinal disorders can cause sialorrhea. Among medications, antipsychotics and cholinergic agonists used in Alzheimer’s dementia are the usual offenders .

Clozapine is the most likely agent to cause significant sialorrhea though other antipsychotics can increase salivation to a small degree. Even though clozapine has significant anticholinergic properties, it increases salivation by cholinergic agonism . The effect is thought to result from an imbalance between agonism and antagonism between different muscarinic receptors in salivary gland tissue. It may also be related to its alpha adrenergic antagonistic properties, which leads to unopposed beta adrenergic stimulation.

Sialorrhea occurs in 30–80% of patients on clozapine [1]. It occurs early in treatment and is usually more copious at night. Reduction of dose does not reliably improve the symptom.

In addition to the obvious inconvenience, untreated sialorrhea can cause mouth irritation and salivary gland enlargement .