It seems harmless: You take a multivitamin, so why not toss one into your dog's breakfast to give him all the added benefits? But the reality is that a human's multivitamin is not made for animals, and, in fact, some of the percentages of vitamins in our supplements can be toxic to them. Additionally, chewable vitamins often contain sweeteners that are perfectly fine for us, but are toxic to dogs. If you want to give your dog an immunity boost, look for supplements that are specifically formulated just for canines.