The yellowish light or dark gray color of the teeth is due to the fact that enamel and dentin are formed simultaneously on opposite sides of a basement membrane, so that the same dose can form a yellow layer in both tissues; however, deposition is four times higher in dentin than in enamel, and in enamel only as diffuse non-banded pigment. This is due to the fact that dentin apatite crystals are small and have a larger total surface area than enamel apatite crystals, thus allowing the dentin to absorb far more tetracycline than the enamel. Because the yellow layer is wavy and cap-like, roughly resembling the shape of a tooth, the staining caused by a single dose can be seen on most of the surface of a tooth; whereas long intervals of repeated doses do not appear as interval horizontal staining. Along with the tooth staining, there is also staining of the bone tissue, but the latter can be gradually removed with the physiological metabolic activity of the bone tissue; however, the tooth staining is permanent. In addition, tetracycline can also cause staining of milk teeth in the mother through the placenta. To prevent the occurrence of yellowish light or dark gray teeth, tetracyclines should not be used in pregnant and lactating women, or in children under 8 years of age, and the amount of fluoride in drinking water should be controlled. The prevention of tetracycline teeth is as simple as not using tetracycline drugs in women who are pregnant or breastfeeding, or in children under 8 years of age. Tetracyclines have an affinity for calcium ions in the body and combine with calcium to form a yellow, stable tetracycline calcium complex. If these drugs are taken during the calcification stage of crown development, the complex will be deposited in the dentin, causing yellowing and yellow staining of the grown teeth, as well as affecting the calcification of the teeth and causing underdevelopment of the enamel. Tetracycline can also cause staining of milk teeth through the placenta when taken by pregnant women. In addition, tetracycline is very sensitive to light and can be discolored by both sunlight and ultraviolet light, so tooth discoloration will gradually deepen over time. Tetracyclines are contraindicated from 4 months of embryonic age until children reach the age of 7 to 8 years for tooth replacement.