not harmful, just not healthy. dog foods now-a days are filled with fillers like corn, Oat Hulls, Rice Hulls, Wheat Middlings, and Cellulose. they have tons of grains and veggies, and very little (if any) meat. most of the time they only have meat-by products, and although they aren't bad for your dog to eat, then need the nutrients from the actual meat part of the animal too.
they tend to have flavor enhancers like Sugar, Sorbitol, Cane Molasses, Fructose, and Corn Syrup are all sweeteners. As yummy as candy and sweet stuff is, dogs donít need that junk. Sweeteners can lead to obesity, Diabetes, and tooth decay.
they add chemicals to the dog food (new research is coming out saying these chemicals have been linked to cancer)
artifical coloring, preservitives, and synthetic vit K are all chemicals that have been linked to cancer.