Hi, You seem to have chronic cheilitis. You feel that black line is there on lower lip. I think it may be crust occuring off and on.There may be erythema, dryness,mild scaling, fissuring and crusting. You might have mild pain, burning and itching. There may be some cause. Lip licking might exacerbate the disease. Sunrays exposure, cosmetics,lipstiks tooth paste, dusts...etc might be resposible. Vitamin B complex deficiencies,anaemia or nutritional inadequacy might cause the disease...You consult dermatologist for firm diagnosis and perfect treatment of its cause. Have blood tests to rule out any internal disease or vitamin deficiencies. You may take folic acid 5 mg thrice a day with iron tab. Antibiotics may be needed if there is secondary bacterial infection. Apply antibitic cream with mild steroid like mometasone. You avoid soap,cosmetics and lipstik for good and fast result. Avoid bad habit of lip licking,if it is there. Do not injure the lip skin with your nails. You might get good result after few weeks. I hope this would help you in your treatment. Thanks. Dr. Ilyas Patel MD