The causes of dark spots on lips include lip inflammation, dark spot polyp syndrome, other factors and so on. 1. Lip inflammation: inflammation caused by the lips will result in localized pigmentation, leading to dark spots on the lips, and there will also be a series of manifestations such as peeling, blistering and so on. 2. Black spot polyp syndrome: black spots on the lips, accompanied by abdominal pain, diarrhea and blood in the stool and a series of symptoms, can be considered as black spot polyp syndrome. 3. Other factors: Prolonged exposure to ultraviolet rays, use of cosmetics containing harmful chemicals, lack of sleep and excessive mental stress may lead to dark spots on the lips. There are many reasons that lead to dark spots on lips. If you have dark spots on lips, it is recommended to go to the hospital and find out the reasons under the guidance of the doctor.