Dry mouth
Xerostomia, most often known simply as dry mouth, is a very common effect of a wide variety of substances, especially diuretics (which increase the rate of urination). Psychoactive stimulants, especially with amphetamines at higher doses, are known to reduce glandular release of saliva, which can cause tooth decay over time. Ensuring proper hydration and electrolyte balance can help mitigate this symptom. Xerostomia is diagnosed based on the person's experience of the symptom, rather than the actual dryness of one's mouth. Other than stimulants, anticholinergic drugs such as tropane alkaloids like Scopolamine or Diphenhydramine, cannabinoids such as THC (especially if the person is smoking cannabis), opioids, antidepressant-class drugs such as the Tricyclic Antidepressants, SSRI's, as well as Lithium - any one of these may lead to dry mouth - in combination the chance of this effect manifesting increases manyfold, and should be watched. Benzodiazepines are also known to bring about dry mouth.
Acute Xerostomia is the subjective effect of having a dry mouth, and is most often simply the result of dehydration. It is also a common side effect of many substances, especially polypharmacy, as the result of drug interactions, especially the combination of more than one drug which can produce or amplify already present experience of a dry mouth.
Chronic Xerostomia or Dry Mouth Syndrome is the regular and/or consistent experience of having a dry mouth. Other than physiological causes like decreased glandular saliva production or Sjögren's syndrome, the most common cause of chronic Xerostomia is polypharmacy and/or the combination of more than one drug that brings about the effect.
Treatments exist, however most are as mouthwashes containing Cevimeline or similar substance which stimulates the release of saliva. Drugs like Cevimeline are muscarinic agonists, selective to M1 and M3 muscarinic-acetylcholine receptors. A problem noted with repeated use of such treatments is the overall worsening of the Xerostomia over time. If the body adjusts to having chemical assistance increasing saliva, cessation of treatment with the chemical can lead to physiological dependence wherein there is a rebound effect upon abrupt stopping of treatment. In general, if the cause of dry mouth is from a psychoactive drug or combination, medical treatment is not recommended. As a basic harm reduction practice, one should stay consistently well hydrated, avoid mouth breathing, and limit or stop drug use if the effect becomes uncomfortable or unmanageable.
Dry Mouth should not be considered anything but entirely benign unless the effect is chronic and continues to bother the person experiencing it.