Oral ulcers are sores that appear on the soft tissues in the mouth. They may be red, white, yellow or grey in appearance. Despite being painful, most oral ulcers are harmless and will heal within 10-14 days. On rare occasions, some ulcers do not heal and may need to be biopsied and examined under the microscope. Please visit the dentist if you have noticed an ulcer that has persisted for more than four weeks.