Dehydration, radiotherapy involving the salivary glands, chemotherapy and several diseases can cause reduced salivation (hyposalivation), or a change in saliva consistency and hence a complaint of xerostomia.

Hyposalivation is a clinical diagnosis that is made based on the history and examination,[1] but reduced salivary flow rates have been given objective definitions.

These include: There may also be other systemic signs and symptoms if there is an underlying cause such as Sjögren's syndrome,[1] for example, joint pain due to associated rheumatoid arthritis.

A reduction in saliva production to about 50% of the normal unstimulated level will usually result in the sensation of dry mouth.

[8] Salivary flow rate is decreased during sleep, which may lead to a transient sensation of dry mouth upon waking.

[11] During periods of stress, our body responds in a ‘fight or flight’ state that will interfere with the saliva flow in the mouth.

[1] However, polypharmacy is thought to be the major cause in this group, with no significant decreases in salivary flow rate being likely to occur through aging alone.

Secondary Sjögren's syndrome is identical to primary form but with the addition of a combination of other connective tissue disorders such as systemic lupus erythematosus or rheumatoid arthritis.

Radiotherapy for oral cancers usually involves up to 70 Gy of radiation, often given along with chemotherapy which may also have a damaging effect on saliva production.

Hormonal disorders, such as poorly controlled diabetes, chronic graft versus host disease or low fluid intake in people undergoing hemodialysis for renal impairment may also result in xerostomia, due to dehydration.

[20] Xerostomia may be a consequence of infection with hepatitis C virus (HCV) and a rare cause of salivary gland dysfunction may be sarcoidosis.

Despite a close association with COVID-19, xerostomia, dry mouth and hyposalivation tend to be overlooked in COVID-19 patients and survivors, unlike ageusia, dysgeusia and hypogeusia.

This test is simple and noninvasive, and involves measurement of all the saliva a patient can produce during a certain time, achieved by dribbling into a container.

Where the symptom is caused by hyposalivation secondary to underlying chronic disease, xerostomia can be considered permanent or even progressive.

[3] A systematic review compromising of 36 randomised controlled trials for the treatment of dry mouth found that there was no strong evidence to suggest that a specific topical therapy is effective.

[8] Sugar free chewing gum increases saliva production but there is no strong evidence that it improves symptoms.

[8] There is a suggestion that intraoral devices and integrated mouthcare systems may be effective in reducing symptoms, but there was a lack of strong evidence.

[8] A systematic review of the management of radiotherapy-induced xerostomia with parasympathomimetic drugs found that there was limited evidence to support the use of pilocarpine in the treatment of radiation-induced salivary gland dysfunction.

[6] It was suggested that, barring any contraindications, a trial of the drug be offered in the above group (at a dose of five mg three times per day to minimize side effects).

[6] Another systematic review showed, that there is some low-quality evidence to suggest that amifostine prevents the feeling of dry mouth or reduce the risk of moderate to severe xerostomia in people receiving radiotherapy to the head and neck (with or without chemotherapy) in the short- (end of radiotherapy) to medium-term (three months postradiotherapy).

[29] A 2013 review looking at non-pharmacological interventions reported a lack of evidence to support the effects of electrostimulation devices, or acupuncture, on symptoms of dry mouth.