Icosin is a combination of vitamin AD and cod liver oil is also a combination of vitamins A and D. The difference between the two is the ratio of vitamin A to vitamin D content. Icosin is a standardized dose of vitamin A to D ratio. Cod liver oil, which is derived from different fish products, has different ratios of vitamins A and D. Often natural cod liver oil AD is not in the proper ratio. When using regular cod liver oil for vitamin D supplementation, vitamin A tends to be toxic once the intake of vitamin D is assured, and if the dose of vitamin A is just maintained at a normal level, the amount of vitamin D may be low. So in between, if you need to use it, Icosin is recommended.