Symptoms

Mouth ulcers often begin with a tingling or burning sensation at the site of the future sore. In a few days they often progress to form a red spot or bump, followed by an open ulcer.

The mouth ulcer appears as a white or yellow oval with an inflamed red border. Sometimes a white circle or halo around the lesion can be observed. The grey, white, or yellow coloured area within the red boundary is due to the formation of layers of fibrin, a protein involved in the clotting of blood. The ulcer, which itself is often extremely painful when agitated, may be accompanied by a painful swelling of the lymph nodes below the jaw, which can be mistaken for toothache.

Mouth ulcers are classified according to the diameter of the lesion. Minor aphthous ulcers, the most frequent type, are less than 10 mm in diameter and typically heal in 1-2 weeks without scarring. Major aphthous ulcers are greater than 10 mm in diameter and are incredibly painful. They usually take more than a month to heal and frequently leave a scar. Herpetiform ulcers are small, usually less than 1-2 mm in diameter, and form clusters. They typically heal in less than a month without scarring.

[source: wikipedia]