Pathophysiology: The main effect of tetracycline on teeth is coloring, sometimes accompanied by enamel hypoplasia. The influence of tetracycline on tooth coloring and enamel hypoplasia is related to the following factors: the color of tetracycline drugs, such as copper yellow for norchlortetracycline and lemon yellow for oxytetracycline. Degrade the color of tetracycline surface, because tetracycline is sensitive to light and will change color under ultraviolet radiation or exposure. Tetracycline in dentin changes the color of dentin in different degrees because of the deep overflow of the binding site. The closer the colored area is to the enamel dentin boundary, the easier it is to be colored. It is related to the structure of enamel itself. When the enamel is severely underdeveloped and completely lost, the colored dentin is obviously exposed. Early use of tetracycline can cause tooth coloring and enamel hypoplasia, but it will not cause tooth coloring after 6-7 years old.