Fish oils are similar to cod liver oils in their omega-3 profile, so if that’s why your doctor is recommending fish oil, either will do. However, the two differ in the part of the fish used. Fish oils are from the flesh of tuna, cod, and other deep-sea fish, whereas cod liver oil is only derived from the livers of codfish. Cod liver oil has the added benefit of containing bioavailable vitamins D and A, whereas regular fish oil contains little of these. As always, consult with your doctor or healthcare practitioner before starting a supplement.