Dry mouth and eyes can be caused by allergies, dehydration, certain medications, or an illness known as Sjogren's syndrome. Seasonal allergies or environmental allergies can both cause dryness in the mouth and eyes, and sometimes eyedrops or allergy medications can help to temporarily relieve the problem. Specific irritants should be identified if possible, in order to be avoided in the future. Sjogren's syndrome, one of the leading causes of persistent dry mouth and eyes, is an autoimmune disease; this means the immune system is essentially responding to a nonexistent problem in the body, making the immune system attack itself.

Though Sjogren's syndrome commonly affects the salivary glands and tear ducts, it can also affect other areas of the body, causing pain in the joints or skin rashes, among other issues. A doctor can help to diagnose and treat Sjogren's syndrome, which may help to relieve the symptoms of dry mouth and eyes. If dry mouth and eyes are not caused by allergy or illness, however, dehydration may be the problem. It is important for people to drink enough water throughout the day both for overall health and to prevent dryness.

The flu can sometimes cause dry mouth and eyes, as can any illness that is accompanied by a fever. Medications are also one of the leading causes of dry mouth and dyes. Drugs such as antihistamines, decongestants, antidepressants, and blood pressure medications, among others, can lead to persistent dryness. For some people, dryness of the mouth and eyes is a persistent problem without a clear cause; regular use of lubricating eye drops may help to prevent redness, dryness, or itching in the eyes. Breathing through the mouth can also cause the mouth to become dry, so sufferers should breathe through the nose as much as possible.

Sometimes, the environment can lead to dry mouth and eyes. Central air conditioning or heating systems, or buildings such as offices or hospitals where air is frequently recycled, can all lead to dryness. The only treatment is to stay hydrated throughout the day, and to use eyedrops if the eyes become dry and painful. Some people find that using a humidifier at night in the home can help to prevent dryness as well; small bedroom humidifiers and can be filled with water and run all night, which can be very beneficial. They help to add water to the air, which helps to prevent dryness in the mouth and eyes.