The unpleasant sensation of dry mouth is thought to start when there is a decrease of 45 per cent in normal salivary flow1 or a change in the normal composition of saliva. However, the condition is readily missed because other symptoms encountered in this patient population are often perceived to be more important.2

Dry mouth can present as dysphagia, oral discomfort, loss of taste and prolonged chewing, and can have a profound impact on quality of life.3 It can cause difficulties in speaking and oral discomfort can be more profound for denture wearers. Patients with dry mouth can develop cracks and fissures in the oral mucosa and halitosis can be another unpleasant feature.4

Xerostomia can result in malnutrition and even decreased social interaction. Patients with dry mouth also have a higher risk of developing dental caries. Decreased levels of saliva predispose patients to an overgrowth of Candida albicans, causing oral thrush.5 In patients who have received ionising radiation therapy to the head and neck, rampant dental caries has been seen just weeks after treatment.6

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Approximately 90 per cent of saliva is produced by the major salivary glands, the parotid, submandibular and sublingual glands. The remaining 10 per cent is secreted by hundreds of minor salivary glands beneath the oral mucosa.7 In healthy individuals, saliva output is said to be about one litre per day.8

Causes of xerostomia
The underlying causes of a dry mouth should be established by detailed history-taking and examination. Often, systemic medication can be a main cause or contributor to the development of xerostomia, by either decreasing salivary output (hyposalivation) or altering the composition of saliva.9

More than 500 medicines are thought to cause a dry mouth,10 so a full medication history is important, including recent chemotherapy regimens. The risk of xerostomia increases with the number of medications a patient is taking.11

Box 1 lists some of the most common drugs causing dry mouth. For example, opioids can cause dry mouth, as can tricyclic antidepressants, antihistamines and diuretics. Drugs with antimuscarinic properties can significantly decrease salivary output. Nasal obstruction can lead to mouth breathing, which can lead to dryness of the mouth, lips and throat.

It is important to assess the patient’s hydration status and fluid intake, as well as their mental state. Anxiety can be a major contributing factor of dry mouth because of autonomic hyperactivity. Caffeine and nicotine intake can also contribute to dry mouth.

Xerostomia is common in patients dying of cancer. About 30 per cent of patients with cancer have problems with a dry mouth and there is an increase in severity with more advanced disease.12 Radiotherapy to the mouth, head, or neck can cause severe xerostomia due to hyposalivation. Salivary gland tissue is very sensitive to ionising radiation and dosages >30Gy can change salivary function irreversibly, depending on the location of the salivary glands relative to the radiotherapy field.13

Dry mouth can be caused by a wider variety of conditions and does not have to be a consequence of existing cancer and its treatment regimens. There is an association with ageing, with prevalence tending to increase with age and accounting for approximately 30 per cent of the population over the age of 65.14 Diabetes mellitus can give patients dry mouth,15,16 as can other systemic conditions, such as thyroid gland disorders,17 rheumatoid arthritis18 and Sjogren’s syndrome.19 Sarcoidosis20 and haemochromatosis are examples of rarer conditions that can lead to destruction of the salivary glands. Graft-versus-host disease is also thought to contribute to dry mouth symptoms.21