Mouth ulcers are sores that appears in the mouth, often on the inside of the cheek . Mouth ulcers also known apthous ulcers can be painful when eating, drinking or brushing.occasional mouth ulcers are usually harmless and clear up on their own.consult your dentist if they last longer than 3 weeks or keep comming back.the exact cause of the mouth ulcers is unknown.stress or tissue injury the cause of simple mouth ulcers.complex mouth ulcers causes is vitamin B-12 deficiency,zinc,folic acid or iron deficiency and gastrointestinal tract disease.