When the full thickness of the epithelium is penetrated (ulceration), the lesion becomes covered with a fibrinous exudate and takes on a yellow-grey color. Because an ulcer is a breach of the normal lining, when seen in cross section, the lesion is a crater. A "halo" may be present, which is a reddening of the surrounding mucosa and is caused by inflammation. There may also be edema (swelling) around the ulcer. Chronic trauma may produce an ulcer with a keratotic (white, thickened mucosa) margin. Malignant lesions may ulcerate either because the tumor infiltrates the mucosa from adjacent tissues, or because the lesion originates within the mucosa itself, and the disorganized growth leads to a break in the normal architecture of the lining tissues. Repeat episodes of mouth ulcers can be indicative of an immunodeficiency, signaling low levels of immunoglobulin in the oral mucous membranes. Chemotherapy, HIV, and mononucleosis are all causes of immunodeficiency/immunosuppression with which oral ulcers may become a common manifestation. Autoimmunity is also a cause of oral ulceration. Mucous membrane pemphigoid, an autoimmune reaction to the epithelial basement membrane, causes desquamation/ulceration of the oral mucosa. Numerous aphthous ulcers could be indicative of an inflammatory autoimmune disease called Behçets disease. This can later involve skin lesions and uveitis in the eyes. Vitamin C deficiency may lead to scurvy which impairs wound healing, which can contribute to ulcer formation. For a detailed discussion of the pathophysiology of aphthous stomatitis, see Aphthous stomatitis#Causes.
Other examples of systemic disease capable of causing mouth ulcers include lupus erythematosus, Sweet syndrome, reactive arthritis, Behçet syndrome, granulomatosis with polyangiitis, periarteritis nodosa, giant cell arteritis, diabetes, glucagonoma, sarcoidosis and periodic fever, aphthous stomatitis, pharyngitis and adenitis.The conditions eosinophilic ulcer and necrotizing sialometaplasia may present as oral ulceration. Macroglossia, an abnormally large tongue, can be associated with ulceration if the tongue protrudes constantly from the mouth. Caliber persistent artery describes a common vascular anomaly where a main arterial branch extends into superficial submucosal tissues without a reduction of diameter. This commonly occurs in elderly people on the lip and may be associated with ulceration. Treatment
Treatment is cause-related, but also symptomatic if the underlying cause is unknown or not correctable. It is also important to note that most ulcers will heal completely without any intervention. Treatment can range from:

Smoothing or removing a local cause of trauma
Addressing dry mouth
Substituting a problem medication or SLS-free toothpaste
Maintaining good oral hygiene and use of an antiseptic mouthwash or spray (e.g. chlorhexidine), which can prevent secondary infection and therefore hasten healing
A topical analgesic (e.g.