It is the most common painful complication associated with systemic chemotherapy (5-fluorouracil, methotrexate) or radiotherapy to the head and neck. It is seen typically 5–7 days post chemotherapy and is mostly seen in haematology patients having ablation therapy. Immunoglobulins are decreased in saliva which may increase the risk of mucositis. It is more common in those that receive accelerated fractionated irradiation.