There are many causes of corners of the mouth sores. When the weather is cold and dry, the corner of the mouth is often licked with the tongue, resulting in wet corners of the mouth, which can easily cause corners of the mouth sores. In addition, they can also be caused by a variety of diseases, with corns of the mouth and herpes simplex being the most common. 1. Corns of the mouth: Mostly caused by malnutrition and lack of vitamin B2, they are common in many children and can manifest as unilateral or bilateral sores, inflammation, redness and pain in the corners of the mouth, which can be improved by supplementing with vitamin B2. It may also be caused by local bacterial and fungal infections, such as Candida infection, which is manifested by vesicles in the corners of the mouth with white secretions, also known as sores, and can be treated with anti-inflammatory and antifungal treatments. 2. Herpes simplex: Herpes virus infection causes sores in the corners of the mouth, and patients can have rice-sized clusters of blisters in the corners of the mouth, which can also be ulcerated, vesiculated and painful. After antiviral treatment under the guidance of a doctor, generally 7-10 days to heal; 3, oral mucosa ulcers: ulcers that occur in the corner of the mouth or close to the corner of the mouth, there will also be local ulcers and pain performance at the corner of the mouth. At this time, you can use watermelon cream and other drugs for treatment.