Various types of mouth sores can appear in or around the mouth for a number of reasons. Injuries to the mouth, such as a bite to the inside of the cheek, are common sources of mouth sores. Chemical or allergic irritation can cause blisters and ulcers to form, and cold sores or fever blisters will form as the result of a viral infection. The causes of mouth sores called canker sores are not fully understood, but they are believed to be related to the immune system. Many other conditions can trigger mouth sores as well, but these are some of the most common.

Injury is perhaps the simplest cause for mouth sores; a cut or scrape to the inside of the mouth causes injury in the form of an ulcer or blister. Blistering, fluid-filled sores in the mouth are referred to as vesicles or bullae, depending on the sore’s size. Chemical burns and allergic reactions to food or medications also can create mouth sores. These sores often are quite painful, especially when eating salty or acidic foods, until healing has had a chance to take place.

Viral infection is the most common cause of mouth sores. The familiar cold sores and fever blisters are caused by herpes simplex virus (HSV) type I and are highly contagious, often passed on through contact with infected skin or saliva. HSV infections are perhaps the best known source of this type of sore, but other viral, fungal and bacterial infections such as syphilis and herpes zoster, the virus which causes chicken pox and shingles, also can trigger blisters and ulcers in and around the mouth. Bacterial infections of the teeth or gums can cause an abscess or cellulitis and are potentially serious.

Canker sores, also known as aphthous stomatitis, are another very common type of sore in the mouth. These recurring sores are not contagious and seem to be triggered by an immune system reaction, although the actual cause of canker sores is unknown. The canker sores form as oval ulcers with a white center. They are painful but usually disappear within two weeks without complication. Larger sores, however, can take longer to heal and can cause scarring.

Treatment for these mouth sores varies with the cause, but many require only time to heal. An anesthetic mouthwash or lozenge might help manage the pain. Frequent cleaning with a soft toothbrush also is recommended to help prevent infection. Medicated gels, chemical or laser treatments might also be recommended by a doctor.