Sores or localized abnormalities inside the mouth can arise from a number of causes. Mouth sores can occur on the tongue, gums, lips, or inside the cheeks. Bite injuries to the tongue or inside of the cheek are a common cause of mouth sores. Also commonly, mouth sores represent aphthous ulcers, also known as canker sores or aphthous stomatitis. These shallow, painful mouth ulcers often occur in susceptible individuals during times of stress, infection, or changes in immune status. Certain medications (for example, methotrexate [Trexall]) can cause canker sores as well as deficiencies in some B vitamins (1, 2, 6, and 12), iron, folic acid, and zinc. However, irritation, injury, burns, or infection of any of the tissues in the oral cavity can also lead to mouth sores. Sores in the mouth can occur with certain systemic (affecting multiple locations within the body) diseases such as Crohn's disease, Behçet's syndrome, and systemic lupus erythematosus (SLE). Rarely, mouth sores are among the initial signs of oral cancers. Herpes simplex virus infection causes so-called cold sores, which are typically located on the lips, but they can also occur on the gums.

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