Actually though - you CAN give your dog the type of cod liver oil which humans take. Our vet recommended it when our old dog had a touch of arthritis, and it worked! He told us to put a teaspoon of the oil over Monty's usual food, once a day, and after a few weeks, there was a marked improvement.
I used to work in an animal feeds shop and my boss swore blind that the cod liver oil we sold for animal consumption was exactly the same as that sold for human consumption, just packaged differently.
Well, I may be wrong, but I thought Cod liver oil was a waste product of the fishing industry. As the liver of any animal is a filter the filter is generally polluted, and how can the organ of a dead fishes liver be of any benefit?
I prefer the omega fatty acids, and evening primose oil etc.