The most common reason for darkening lips is cyanosis caused by lack of oxygen, in addition to cold, smoking, certain digestive system diseases, poor peripheral circulation, etc., may also change the original red color of the lips. The color of human skin and mucous membrane changes with the color of blood circulation. The red color of blood is due to the hemoglobin contained in red blood cells that are bound with oxygen, while the color of hemoglobin that is not bound with oxygen is dark red. The main causes of cyanosis caused by blood hypoxia are congenital vascular malformation, pulmonary arteriovenous fistula, respiratory system diseases, hemolytic anemia, nitrite poisoning, etc. The capillaries of the lips are extremely rich, and when hypoxia causes a large increase in the amount of hemoglobin that is not combined with oxygen, the color of the lips will deepen. The lips will also become darker when the capillaries spasm and contract in cold conditions and the amount of blood in the blood vessels decreases, in addition, smoking, drug poisoning and chemical poisoning will also lead to dark lips.