Causes, Symptoms & Remedies of Mouth Ulcers A mouth ulcer is a reddish boil or sore occurring at the inner regions of the cheek area. It occurs due to the loss or erosion of a part of the delicate tissue that lines the mucous membrane. Mouth ulcers are quite bothersome and occasionally painful as they make simple eating and drinking seem like a task. However, they are not so serious as they subside on their own within a week or 10 days. In certain cases though, where a mouth ulcer doesn’t seem to go away on its own even after so many days, it’s time to see a doctor. Causes of Mouth Ulcer The most common cause of mouth ulcers is accidentally biting the insides of your cheeks while talking or chewing on something or even in your sleep. Other causes include poor oral hygiene, aphthous ulceration, reactions to certain medications, skin rashes in the mouth, viral, bacterial and fungal infections, chemicals, some systemic medical diseases and, rarely, malignancy. It coul