If you leave tapeworms untreated in dogs, they can cause several adverse effects. The first and most common is the development of anemia. The tapeworms eat blood from the dog, but they also absorb some of it themselves. If they are left untreated, they will continue to feed on their host's blood until they die off, but this means that the dog's body doesn't have enough red blood cells to function properly. Another common effect is worms in your dog's stool. The worms that cause tapeworm infection are tiny and white, so you might not see them right away when you look at your dog's poop.