Cod Liver Oil is a rich source of Omega-3 fatty acid, one of the primary types of essential fatty acids (EFA's). Essential fatty acids are considered "essential" because they are the main structural components of the body's cell membranes, but cannot be synthesized by the body and must be obtained from the diet.
The Tufts University School of Veterinary Medicine does not recommend Cod Liver Oil for dogs, as the dose required to obtain an optimal level of omega-3s could contain toxically high levels vitamins A and D.