Many of us take vitamins and supplements on a daily basis to help us feel better or get the necessary nutrition we may be lacking. So it seems natural we could give these same natural supplements to cats and dogs to help them be healthy, right? Not so fast. According to the Food and Drug Administration, if your pet already eats a balanced diet, giving him more vitamins and minerals isn't necessarily a beneficial thing. Too much calcium can actually cause skeletal problems for large-breed puppies; vitamin A can harm blood vessels and cause dehydration; and vitamin D -- which gives humans energy -- can force your pooch to stop eating and his muscles to atrophy. Instead of risking it, ask your vet.