A red patch on the mouth may be caused by lipitis, allergies and other reasons. 1. Lipitis: When suffering from lipitis, the phenomenon of lip swelling may occur, and the lips will have the symptom of congestion, which can be manifested as a red patch. 2. Allergy: When the mouth comes into contact with allergens, it can cause an allergic reaction, which can appear as redness and swelling on the mouth. It is recommended that the person with a red mouth go to a regular hospital in a timely manner, to clarify the cause of the disease, under the guidance of the doctor for standardized treatment, so as not to delay the condition of the body to bring about adverse effects. Also need to pay attention to ensure comprehensive and balanced nutrition, try to stay away from allergens, avoid spicy and stimulating food, so as not to aggravate the condition.