Can vitamins be poisonous to dogs?

Vitamins and Supplements can be either harmful or benficial to dogs. Just as in humans, each dog is different and there are differences in genetics and chemistry. That old saying; "one man's medicine is another man's poison" is applicable. However, unless a dog consumed large quantities of a vitamin or supplement it is unlikely there will be any significant harm. If a large quantity of any substance is ingested, in either a human or animal, toxicity is possible and depending on the substance, consequences may vary.