Dry mouth: The condition of not having enough saliva to keep the mouth wet. This is due to inadequate function of the salivary glands. Everyone has dry mouth once in a while when they are nervous, upset or under stress. But if someone has a dry mouth most all of the time, it can be uncomfortable and lead to serious health problems.
Dry mouth can cause difficulties in tasting, chewing, swallowing, and speaking. If it goes untreated, severe dry mouth can also lead to increased levels of tooth decay and infections of the mouth such as thrush. Severe dry mouth is not a normal part of aging. It can be a clue to systemic diseases such as Sjogren syndrome, systemic lupus erythematosus, rheumatoid arthritis, scleroderma, sarcoidosis, amyloidosis, and hypothyroidism. Some medications can also cause dry mouth.

If you're not producing enough saliva, you may notice these signs and symptoms all or most of the time:
Dryness or a feeling of stickiness in your mouth
Saliva that seems thick and stringy
Bad breath
Difficulty chewing, speaking and swallowing
Dry or sore throat and hoarseness
Dry or grooved tongue
A changed sense of taste
Problems wearing dentures
In addition, dry mouth may result in lipstick sticking to the teeth.

Medications.   Hundreds of medications, including many over-the-counter drugs, produce dry mouth as a side effect. Among the more likely types to cause problems are some of the drugs used to treat depression, high blood pressure and anxiety, as well as some antihistamines, decongestants, muscle relaxants and pain medications.
Aging.     Many older people experience dry mouth as they age. Contributing factors include the use of certain medications, changes in the body's ability to process medication, inadequate nutrition, and having long-term health problems.Cancer therapy. Chemotherapy drugs can change the nature of saliva and the amount produced. This may be temporary, with normal salivary flow returning after treatment is completed. Radiation treatments to your head and neck can damage salivary glands, causing a marked decrease in saliva production. This may be temporary or permanent, depending on the radiation dose and area treated.
Nerve damage    An injury or surgery that causes nerve damage to your head and neck area can result in dry mouth.
Other health conditions. Dry mouth can be due to certain health conditions, such as diabetes, stroke, yeast infection (thrush) in your mouth or Alzheimer's disease, or due to autoimmune diseases, such as Sjogren's syndrome or HIV/AIDS. Snoring and breathing with your mouth open also can contribute to dry mouth.
Tobacco and alcohol use.   Drinking alcohol and smoking or chewing tobacco can increase dry mouth symptoms.
Recreational drug use.    Methamphetamine use can cause severe dry mouth and damage to teeth, a condition also known as "meth mouth." Marijuana also can cause dry mouth.

https://www.medicinenet.com/script/main/art.asp?articlekey=24997
https://www.mayoclinic.org/diseases-conditions/dry-mouth/symptoms-causes/syc-20356048
https://www.mayoclinic.org/diseases-conditions/dry-mouth/symptoms-causes/syc-20356048