Most dogs do not need supplements until they are old. However, dogs with special medical needs might require some supplements to lead a long and happy life. Supplements should be added to a dog’s diet as directed by the veterinarian. There are six essential nutrients that your dog needs to have a healthy life. Those nutrients include water, protein, fats, vitamins, carbohydrates and minerals. If the vet feels that the dog does not consume enough nutrients through regular checkups, they will provide you with a prescription for dog supplements.