There are several causes of dry mouth, also called xerostomia. These include:
Side effect of certain medications. Dry mouth is a common side effect of many prescription and nonprescription drugs, including drugs used to treat depression, anxiety, pain, allergies, and colds (antihistamines and decongestants), obesity, acne, epilepsy, hypertension (diuretics), diarrhea, nausea, psychotic disorders, urinary incontinence, asthma (certain bronchodilators), and Parkinson's disease. Dry mouth can also a side effect of muscle relaxants and sedatives.
Side effect of certain diseases and infections. Dry mouth can be a side effect of medical conditions, including Sjögren's syndrome, HIV/AIDS, Alzheimer's disease, diabetes, anemia, cystic fibrosis, rheumatoid arthritis, hypertension, Parkinson's disease, stroke, and mumps.
Side effect of certain medical treatments. Damage to the salivary glands, the glands that produce saliva, for example, from radiation to the head and neck and chemotherapy treatments for cancer, can reduce the amount of saliva produced.
Nerve damage . Dry mouth can be a result of nerve damage to the head and neck area from an injury or surgery.
Dehydration . Conditions that lead to dehydration, such as fever, excessive sweating, vomiting, diarrhea, blood loss, and burns can cause dry mouth.
There’s an old Blues song titled, "You don’t miss your water til the well runs dry." That’s certainly true with saliva. Most people don’t think of it until the moisture in their mouths runs dry, and that’s what makes xerostomia so irritating. Recovering cancer patients wonder, "Why is my mouth so dry?" But the dryness is more than an irritation. It can be a significant problem that impairs a person’s ability to chew, swallow, and even speak. In addition, dry mouth can lead to oral infections, such as tooth decay and Candidiasis.
For some people, yes. If tissue remains that is water permeable and capable of secretion, compounds such as pilocarpine can stimulate the glands to produce more saliva. The problem is most patients have salivary glands that have stopped working.
The radiation leaves the glands water impermeable. Let me explain. Salivary glands kind of look like a bunch of grapes attached to a stem. The grapes are called acinar cells. In people with working salivary glands, water enters the acinar cells, where myriad proteins are added, and the mixture then flows through the stems for further processing and ultimately exit into the mouth as saliva. In most head and neck cancer patients, the radiation has wiped out the acinar cells. They’re left with a network of water-impermeable stems that have no salivary flow. There is nothing left to stimulate.