Rickets, the administration of concentrated cod liver oil to infants has been accepted by parents. However, some parents are still unclear about how to choose the preparation. There are two types of concentrated cod liver oil preparations. One is an old preparation that has been used for many years, with a vitamin A to D content ratio of 10:1, meaning each milliliter of concentrated cod liver oil contains 50,000 units of vitamin A and 5,000 units of vitamin D. However, children need only 1,000-1,500 units of vitamin A and 400 units of vitamin D daily, making their ratio approximately 3-4:1. If this old preparation is used to prevent rickets, giving 3 drops daily would provide 400-500 units of vitamin D but also 5,000 units of vitamin A, which is 4-5 times the required amount, potentially leading to vitamin A poisoning over time. To overcome this problem, half a dose (150,000 units) of injectable vitamin D can be added to each bottle of cod liver oil, adjusting the vitamin A to D ratio to 3:1. With this mixed preparation, one drop per day provides 1,600 units of vitamin A and 600 units of vitamin D, effectively preventing rickets without causing vitamin A poisoning.