Mouth irritations and oral lesions may present as swellings, spots or sores in the mouth, lips or tongue. There are several types of mouth sores. The most common mouth sores are canker sores, cold sores, leukoplakia and candidiasis (thrush). Mouth sores occur commonly. Oral lesions can cause irritation and pain, and can make eating and speaking difficult. Consult your dentist if your mouth sore persists for more than a week. Your dentist may advice biopsy (tissue taken for testing) to determine the cause if your mouth sores are persistent (to rule out serious diseases such as cancer and HIV).