Mouth ulcers are painful breaks in the lining of the mouth. Aphthous ulcers, also called canker sores, are a common type that is not contagious. Most small ulcers heal on their own within one to two weeks.
What they look and feel like
A typical aphthous ulcer is round or oval with a pale center and a red edge. It may occur inside the lips or cheeks, under the tongue or on the soft parts 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 toothpastes 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 injury.
- Rinse gently with salt water and spit it out.
- Ask a pharmacist about a protective paste, pain-relieving gel or other suitable treatment.
Do not place aspirin directly on an ulcer because it can burn the tissue.
When to seek advice
See a dentist or clinician if an ulcer lasts longer than three weeks, is unusually large, repeatedly returns, spreads, bleeds, becomes more painful or is accompanied by fever, diarrhoea, 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 care for swelling of the mouth or throat, difficulty breathing or swallowing, severe dehydration, or a serious reaction after starting a medicine.
Sources
- MSD Manual: Recurrent Aphthous Stomatitis
- MSD Manual: Mouth Sores and Inflammation
- NHS: Symptoms of mouth cancer
Reviewed: 28 August 2026