Cod liver oil is widely taken to ease the pain and joint stiffness associated with arthritis. It may have a positive effect on heart, bone, as well as helping to repair wounded skin, hair, nails, and teeth.

Cod liver oil and fish oil are similar, but cod liver oil has higher levels of vitamins A and D. A tablespoon (15 ml) of cod liver oil (13.6 g) contains 4080 μg of retinol (vitamin A) and 34 μg of vitamin D.

Cod liver oil also may be an effective complementary measure for long-term treatment of multiple sclerosis and use of cod liver oil during pregnancy is associated with lower risk of Type I diabetes in the offspring (although see adverse effects below). This effect was found only in mothers taking cod liver oil, not in mothers taking multivitamin supplements.

Cod liver oil taken by nursing mothers improves the breast milk by increasing the amount of fatty acids, which promotes brain development, and the amount of vitamin A, which helps prevent infections, but the level of vitamin D is unchanged.