There are plenty of horror stories on the Internet.. They have the same concerns that go along with Rawhides. They are not easily digestible, they are a choking hazard, they can cause fatal impaction, and they aren’t made of quality ingredients. They certainly clean teeth, but they have also been retrieved from a dog’s digestive tract hard and intact days after ingestion! Any chew or bone is going to be a choking or impaction hazard, though, and not every dog is going to have these issues.
But here are the first few ingredients in a Greenie: Wheat flour, wheat protein isolate, glycerin, gelatin, oat fiber, water, lecithin, natural poultry flavor
And here Natural Balance Dental Chews: Pumpkin, Potato Starch, Chicken Meal, Glycerin, Papaya, Natural Chicken Flavor
So, there are definitely better ingredients for your dog out there that will still clean their teeth.