Factors that often cause darkening of teeth include exogenous staining, tetracycline, dental fluorosis, dental caries, and pulp necrosis. 1. Exogenous staining: Smoking, drinking tea, coffee, eating chocolate and other dark-colored foods can cause exogenous staining of teeth, which can make teeth discolored or even black. 2. Tetracycline teeth: due to long-term use of tetracycline drugs, especially pregnant women or infants, over time will lead to discoloration of the teeth, usually yellow, brown, dark gray and so on. 3. Dental fluorosis: Due to excessive fluorine intake from drinking water, food and air for a long time during the period of tooth development, it will lead to underdevelopment of tooth enamel and cause dental fluorosis. Common symptoms of dental fluorosis are chalky streaks on the surface of the teeth, point-like defects, and in severe cases, the teeth are dark brown or black. 4. Dental caries: Poor oral hygiene can easily cause dental caries, which is also commonly known as tooth decay. With the gradual deepening of caries, the tooth body will appear defective, and may also be accompanied by tooth discoloration, black spots on the surface of the tooth, black holes, and so on. 5. Pulp necrosis: In addition to caries, pulp necrosis may also cause teeth to turn black. Pulp necrosis is often caused by the evolution of various types of pulpitis, and may also be caused by dental trauma, informal orthodontic. Darkened teeth should be seen promptly to identify the cause and receive treatment.