Xerostomia, also known as dry mouth, is dryness in the mouth, which may be associated with a change in the composition of saliva, or reduced salivary flow, or have no identifiable cause. This symptom is very common and is often seen as a side effect of many types of medication. It is more common in older people (mostly because this group tends to take several medications) and in persons who breathe through their mouths. 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. Sometimes there is no identifiable cause, and there may sometimes be a psychogenic reason for the complaint. Xerostomia is the subjective sensation of dry mouth, which is often (but not always) associated with hypofunction of the salivary glands. The term subjective xerostomia is sometimes used to describe the symptom in the absence of any clinical evidence of dryness. A drug or substance that increases the rate of salivary flow is termed a sialogogue.​​​​​​​