Red patches in the mouth may be due to trauma, infection, immune-mediated conditions or neoplasia.
Mucositis. This may occur following chemotherapy or radiotherapy and may cause painful red patches throughout the oral mucosa, often associated with areas of ulceration. This usually persists for several weeks after the termination of the chemotherapy or radiotherapy before resolving.
Occasionally contact hypersensitivity (allergic reaction) to materials such as dental materials, toothpaste or dentures may cause redness in the mouth. This usually resolves after removal of the causative material.
Immune-mediated conditions such as oral lichen planus, pemphigoid or pemphigus may cause red areas in the mouth. In particular a red appearance of the gums called desquamative gingivitis may occur.
Fungal infections (candidosis) may cause red areas in the mouth, particularly under dentures which are not being removed at night (denture stomatitis) or on the upper surface of the tongue in the midline (medial rhomboid glossitis). These are treated with general advice and antifungal therapy.
Geographic tongue may cause red areas on the tongue with white margins. This can usually be diagnosed with a clinical examination. This is a benign condition although sometimes a blood test is advisable to exclude nutritional deficiencies.
Sometimes a red tongue may occur in anaemia. If this is suspected than a blood test is advisable.
Erythroplakia. Sometimes a red appearance may occur in the mouth due to unknown reasons. This is called erythroplakia. This is usually associated with smoking. The rate of transformation of erythroplakia to cancer is 5-10%. If this is suspected than a biopsy is indicated to exclude mouth cancer and to assess the need for follow up.
Mouth cancer. Occasionally this may present as a red area in the mouth. If this is suspected than an urgent biopsy will be suggested.