A mouth ulcer is the loss or erosion of part of the delicate tissue that lines the inside of the mouth (mucous membrane). There are multiple causes. The most common cause is injury, such as accidentally biting the inside of your cheek. Other causes include aphthous ulceration, certain medications, skin rashes in the mouth, viral, bacterial and fungal infections, chemicals, some systemic medical diseases and, rarely, malignancy.

In most cases, mouth ulcers are harmless and resolve by themselves within 10 days without the need for any treatment.