Sores inside the mouth develop when the lining membrane is damaged, exposing the underlying tissue. This can result from traumatic injury. Wounds or sores in the corners of the mouth or on the tongue can be caused by a poorly fitting or functioning bit or from trauma from excessive force applied to the bit.

Sores can also be caused by a variety of disease processes, including ingestion of toxins (from plants and other sources) that damage the lining cells. Some viral diseases, including EHV-1 and Vesicular Stomatitis (VS), can also cause oral ulcers. NSAID (bute) toxicity will sometimes cause oral ulcers.

A common cause of sores within the mouth is ulceration around seed heads from hay, which become embedded in the gums and cause ulcers to develop.

Summer Sores can occur around the corners of the mouth. More rarely, different skin conditions like Equine Sarcoid and Squamous Cell Carcinoma occur here, at the junction of the oral mucous membranes and the skin of the lips.