The common causes of black corners of the mouth include physiological factors and pathological factors (common in keratitis, fixed drug rash, herpes simplex and other diseases). 1. Physiological factors: If you usually eat some black food, such as black sesame, black beans, black rice, black fungus, etc., and do not wipe your mouth in time, then over time, the food residue accumulates in the corners of the mouth, which can easily lead to melanin pigmentation, the corners of the mouth black, which does not usually affect your health. 2. Pathological factors (1) keratitis: this disease can be caused by fungi, bacteria, viruses and other pathogenic microbial infections, but also by trauma, allergies, malnutrition and other factors, there will be redness of the skin and mucous membrane of the corners of the mouth, cracking, vesicles, scabs, etc., and do not cure in time, and there may be hyperpigmentation, resulting in the corners of the mouth black. (2) fixed drug rash: this disease is mainly associated with the use of antipyretic and analgesic drugs, sulfonamides, sleeping sedative drugs, antibiotics, etc., often occurring in the corners of the mouth mucous membrane, manifested as erythema bright red or purplish-red, edematous, inflammation of the center of the intense can be formed blisters, the healing of the formation of hyperpigmented spots, the emergence of the dark corners of the mouth. (3) herpes simplex: herpes simplex occurs at the corner of the mouth for herpetic gingivostomatitis, mainly caused by herpes simplex virus infection, manifested as lips, tongue, oral mucosa and gingiva edema and diffuse redness, rapidly occurring clusters of small blisters, lesions will be recovered with hyperpigmentation and other symptoms. It is recommended that patients who suspect that the disease is caused by a timely visit to a regular hospital dermatology department, after a clear diagnosis, timely treatment.