 In some instances , recurrent mouth ulcers may be a manifestation of an allergic reaction . Possible allergens include certain foods ( e.g. , chocolate , coffee , strawberries , eggs , nuts , tomatoes , cheese , citrus fruits , <unk> , <unk> , and highly acidic foods ) , toothpastes , and mouthwashes . Where dietary allergens are responsible , mouth ulcers usually develop within about 12 – 24 hours of exposure . 
