Oral Ulcers. Mouth ulcers can have many different causes from the common to the very rare. Some of the causes include herpes simplex, lichen planus, oral apthae (canker sores), pemphigus vulgaris, and several other conditions. You should have this evaluated by your doctor or dermatologist as the problem can be managed effectively regardless of the cause.
Prevention. Canker sore or more specifically aphthous ulcer can be prevented by avoiding a common foam detergent found in most toothpastes called sodium laurel sulfate. Switching to biotene or other toothpaste not containing this ingredient can be the cure in preventing this painful condition.