Cod liver oil is an oil made from the liver of marine fish. The main components of cod liver oil are fat-soluble vitamin A and vitamin D. In addition to this, gelatin, purified water and glycerin are added. Cod liver oil is used for the prevention and treatment of vitamin A and D deficiencies such as rickets, night blindness and pediatric hand and foot convulsions. The main function of vitamin A is to maintain normal growth, reproduction, vision, epithelial tissue soundness and anti-infection immunity of the body. The main function of vitamin D is to promote the absorption of calcium and phosphorus by the small intestinal mucosa; to promote the reabsorption of calcium and phosphorus by the renal tubules. Vitamin AD: It is a synthetic simple vitamin AD drop, usually with a ratio of 3:1 of vitamin A to D. Note that it is synthetic, not taken from natural fish. This is the most essential difference between it and cod liver oil. Cod liver oil is commonly used to prevent night blindness, corneal softening, etc. It also has a protective effect on epidermal tissues such as the upper mucosa of the respiratory tract. Vitamin AD has the same efficacy as cod liver oil, but is used more for the prevention and treatment of vitamin A and D deficiencies. Doctors generally recommend that babies start supplementing with cod liver oil half a month after birth, starting with one drop per day and gradually increasing to a maximum of five drops. For premature babies, twins, or newborns with digestive disorders, cod liver oil should be added from the second week of life, 5 drops per day, and then changed to 3-5 drops after one month. after 2 years of age, the amount of cod liver oil is gradually reduced.