Common types of mouth sores include cold sores and canker sores. In addition, mouth sores may be the result of injury, or they may be a symptom of a more serious disease such as lupus, cancer or hand-foot-mouth disease, MedlinePlus explains.
Cold sores, which are caused by the herpes simplex virus, appear as blisters that crust over and are highly contagious. They can be triggered by stress, illness, sun exposure and hormonal changes. Canker sores, which are not contagious, may be caused by vitamin or mineral deficiency, immune system weakness, stress and hormonal changes, according to MedlinePlus. Certain drugs, including aspirin, beta-blockers, chemotherapy drugs and sulfa drugs, can also cause mouth sores.