Yellowing of teeth is usually caused by dental fluorosis, tetracycline, dietary factors and poor oral hygiene. 1. Dental fluorosis: Dental fluorosis, also known as mottled enamel, is related to the high fluoride content in drinking water. When the fluoride content in drinking water is 1ppm, it can easily lead to yellowing of teeth. 2. Tetracycline teeth: due to the mineralization stage of tooth development, taking tetracycline drugs will make teeth yellow, which may be related to enamel underdevelopment. 3. Dietary factors: People who drink tea, smoke or chew betel nut for a long time have pigment on the surface of their teeth, especially on the tongue, which cannot be removed by brushing. Pits, grooves and rough surfaces of teeth are also prone to yellowing. 4. Poor oral hygiene: Foreign pigments are first deposited in the mucous membrane and plaque on the surface of the teeth. People with poor oral hygiene are prone to hyperpigmentation where plaque is retained, such as near the gum margins and adjacent surfaces. Making teeth yellow. The cause of yellowing teeth may also be caused by pathologic factors, which can be promptly examined in the dentist to take the most appropriate treatment.