For example...(from an internet search)
You should generally not give supplements to your dogs unless you are an expert in canine nutrition or have been advised to do so by your doctor. Human vitamins can contain a number of additives for color, flavor or binding that is safe for human consumption, but unsafe for pets. Additionally, multivitamins can contain supplements that are toxic to dogs. Vitamins containing iron, for instance, can cause damage to the lining of your dog's digestive system, and may even cause liver or kidney damage. Human vitamins are also formulated for the average weight of a human, which is often much greater than the average dog. As a result, the quantity of supplements in a human vitamin may be much greater than a dog's system can handle, forcing the liver and kidneys to work to filter these excess supplements. Pet food is often formulated to contain all the nutrients a dog needs, and excess supplements from vitamins may overload a dog's system, even if given in very small quantities.