An oral ulcer is the name for the appearance of an open sore inside the mouth caused by a break in the mucous membrane or the epithelium on the lips or surrounding the mouth. The types of oral ulcers are diverse, with a multitude of associated causes including: physical or chemical trauma, infection from microorganisms, medical conditions or medications, cancerous and nonspecific processes. Once formed, the ulcer may be maintained by inflammation and/or secondary infection. Two common oral ulcer types are aphthous ulcers (canker sores) and cold sores (aka fever blisters).