Mouth irritations and oral lesions are swellings, spots or sores on your mouth, lips or tongue. Although there are numerous types of mouth sores and disorders, among the most common are Aphthous ulcers, cold sores, leukoplakia and candidiasis (thrush). These are discussed below. If you have a mouth sore, you're not alone-around one third of all people are affected at some point. Nonetheless, mouth sores, irritations and lesions can be painful, unsightly and can interfere with eating and speaking. Any mouth sore that persists for a week or more should be examined by your dentist. A biopsy (tissue taken for testing) may be advised and can usually determine the cause, ruling out such serious diseases as cancer and HIV.