Canker sores are small, round sores in your mouth. They can be on the inside of your cheek, under your tongue, or in the back of your throat. They usually have a red edge and a gray center. They can be quite painful. They are not the same as cold sores, which are caused by herpes simplex.

Canker sores aren't contagious. They may happen if you have a viral infection. They may also be triggered by stress, food allergies, lack of vitamins and minerals, hormonal changes or menstrual periods. In some cases the cause is unknown.

In most cases, the sores go away by themselves. Some ointments, creams or rinses may help with the pain. Avoiding hot, spicy food while you have a canker sore also helps.

Both fever blisters and canker sores are common across all ages and can be painful. However, they are separate conditions with different causes. Read further to learn how to tell them apart and find out which treatments might help ease the pain.

Canker Sores are painful white or yellow sores that only form inside the mouth, usually on the insides of the cheeks or lips or on the tongue. Canker sores (also called aphthous ulcers) are not contagious.

Food enters the digestive tract through the mouth, where mechanical digestion (by chewing) and chemical digestion (by enzymes in saliva) begin. Within the mouth are the tongue, teeth, and salivary glands, including the parotid, sublingual, and submandibular glands (Figure 24.3). The salivary glands produce saliva, which lubricates food and contains digestive enzymes.

Figure 24.3 (a) When food enters the mouth, digestion begins. (b) Salivary glands are accessory digestive organs. (credit: modification of work by National Cancer Institute)

The structure of a tooth (Figure 24.4) begins with the visible outer surface, called the crown, which has to be extremely hard to withstand the force of biting and chewing. The crown is covered with enamel, which is the hardest material in the body. Underneath the crown, a layer of relatively hard dentin extends into the root of the tooth around the innermost pulp cavity, which includes the pulp chamber at the top of the tooth and pulp canal, or root canal, located in the root. The pulp that fills the pulp cavity is rich in blood vessels, lymphatic vessels, connective tissue, and nerves. The root of the tooth and some of the crown are covered with cementum, which works with the periodontal ligament to anchor the tooth in place in the jaw bone. The soft tissues surrounding the teeth and bones are called gums, or gingiva. The gingival space or gingival crevice is located between the gums and teeth.

Figure 24.4 The tooth has a visible crown with an outer layer of enamel, a layer of dentin, and an inner pulp. The root, hidden by the gums, contains the pulp canal (root canal). (credit: modification of work by Bruce Blaus)

Microbes such as bacteria and archaea are abundant in the mouth and coat all of the surfaces of the oral cavity. However, different structures, such as the teeth or cheeks, host unique communities of both aerobic and anaerobic microbes. Some factors appear to work against making the mouth hospitable to certain microbes. For example, chewing allows microbes to mix better with saliva so they can be swallowed or spit out more easily. Saliva also contains enzymes, including lysozyme, which can damage microbial cells. Recall that lysozyme is part of the first line of defense in the innate immune system and cleaves the β-(1,4) glycosidic linkages between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in bacterial peptidoglycan. Additionally, fluids containing immunoglobulins and phagocytic cells are produced in the gingival spaces. Despite all of these chemical and mechanical activities, the mouth supports a large microbial community.

Canker Sores do not have a known cause. They may be triggered by an injury, stress, smoking, or deficiencies in folic acid, iron, or vitamin B12.

Symptoms of canker sores:

• A round white or yellow sore with a red border inside the mouth, usually on the inside of the lips or cheeks or on the tongue.
• A burning or tingling sensation may occur in the mouth prior to the formation of the sore.

The main way to tell the difference between a fever blister and a canker sore is by location. Fever blisters occur outside the mouth, generally around the border of the lips. Canker sores occur inside the mouth.

They also differ in appearance: Fever blisters are patches of several small fluid-filled blisters, while canker sores are usually single round white or yellow sores with a red border.

Typically, both types of sores go away on their own and treatment is not usually required. There is no known treatment that can permanently cure fever blisters or canker sores and prevent them from recurring. Treatment focuses on minimizing the pain and speeding recovery time.

Canker Sores can be treated with over-the-counter (non-prescription) gels to numb the pain. Avoiding spicy or abrasive foods may also help. Rinsing the mouth with over-the-counter antiseptic medications may help prevent infection of the sore and speed healing.

See a doctor if you have frequently recurring canker sores or if the sores are severe enough to make eating or drinking difficult. If you have recurring canker sores as a result of a vitamin deficiency, treating the deficiency may reduce their frequency.

Both fever blisters and canker sores can have triggers that produce flare ups. Try to identify and then avoid triggers so your flare ups become less frequent.

Tips for Canker Sores

• If you regularly get canker sores, talk to your doctor about testing for food allergies, as allergic reactions can sometimes trigger canker sore outbreaks.