Mouth ulcers are painful breaks in the lining of the mouth. Aphthous ulcers, also called canker sores, are a common type and are not contagious. Most small ulcers heal on their own within one to two weeks.
What they look and feel like
A typical canker sore is round or oval with a pale center and a red border. It may occur inside the lips or cheeks, under the tongue or on other soft areas of the mouth. Tingling or burning can begin before the ulcer appears.
Possible triggers and causes
Minor injury from biting, dental work, braces or a sharp tooth can cause an ulcer. Stress, hormonal changes and certain foods or oral-care products may trigger recurrent ulcers in some people. Frequent or severe ulcers can sometimes be associated with iron, folate or vitamin B12 deficiency, celiac disease, inflammatory bowel disease, immune conditions or medications.
Self-care
- Avoid spicy, acidic, very hot or rough foods while the ulcer heals.
- Use a soft toothbrush and avoid further irritation.
- Rinse gently with salt water and spit it out.
- Ask a pharmacist or dental professional about a protective paste, pain-relieving gel or other appropriate treatment.
Do not place aspirin directly on an ulcer because it can injure the tissue.
When to get medical or dental advice
Arrange an assessment if an ulcer lasts longer than three weeks, is unusually large, repeatedly returns, bleeds, becomes increasingly painful or occurs with fever, diarrhea, unexplained weight loss, skin or genital sores, or difficulty eating and drinking. A persistent red or white patch, lump, neck swelling, swallowing difficulty or unexplained hoarseness also needs assessment.
When to seek urgent help
Seek urgent medical care for swelling of the mouth or throat, difficulty breathing or swallowing, severe dehydration, or signs of a serious allergic or medication reaction.
Sources and further reading
Last reviewed: 28 September 2026.
This information is general and does not replace individual medical or dental assessment or treatment.