Most canker sores are round or oval with a white or yellow center and a red border. They form inside your mouth — on or under your tongue, inside your cheeks or lips, at the base of your gums, or on your soft palate. You might notice a tingling or burning sensation a day or two before the sores actually appear.

There are several types of canker sores, including minor, major and herpetiform sores.

Minor canker sores

Minor canker sores are the most common and:

• Are usually small
• Are oval shaped with a red edge
• Heal without scarring in one to two weeks

Major canker sores

Major canker sores are less common and:

• Are larger and deeper than minor canker sores
• Are usually round with defined borders, but may have irregular edges when very large
• Can be extremely painful
• May take up to six weeks to heal and can leave extensive scarring

Herpetiform canker sores

Herpetiform canker sores are uncommon and usually develop later in life, but they're not caused by herpes virus infection. These canker sores:

• Are pinpoint size
• Often occur in clusters of 10 to 100 sores, but may merge into one large ulcer
• Have irregular edges
• Heal without scarring in one to two weeks

The precise cause of canker sores remains unclear, though researchers suspect that a combination of factors contributes to outbreaks, even in the same person.

Possible triggers for canker sores include:

• A minor injury to your mouth from dental work, overzealous brushing, sports mishaps or an accidental cheek bite
• Toothpastes and mouth rinses containing sodium lauryl sulfate
• Food sensitivities, particularly to chocolate, coffee, strawberries, eggs, nuts, cheese, and spicy or acidic foods
• A diet lacking in vitamin B-12, zinc, folate (folic acid) or iron
• An allergic response to certain bacteria in your mouth
• Helicobacter pylori, the same bacteria that cause peptic ulcers
• Hormonal shifts during menstruation
• Emotional stress

Canker sores may also occur because of certain conditions and diseases, such as:

• Celiac disease, a serious intestinal disorder caused by a sensitivity to gluten, a protein found in most grains
• Inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis
• Behcet's disease, a rare disorder that causes inflammation throughout the body, including the mouth
• A faulty immune system that attacks healthy cells in your mouth instead of pathogens, such as viruses and bacteria
• HIV/AIDS, which suppresses the immune system

Unlike cold sores, canker sores are not associated with herpes virus infections.

Anyone can develop canker sores. But they occur more often in teens and young adults, and they're more common in females.

Often people with recurrent canker sores have a family history of the disorder. This may be due to heredity or to a shared factor in the environment, such as certain foods or allergens.