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Propylene glycol is used to absorb water from foods, people food too! I think the danger comes from consuming too much of it. People vary their diet a lot, so I feel as though if you're eating decently, you won't get too much if it (it's in food like candy). But if you're dog food/treats have them, I feel like your dog could be getting too much of it. Personally, I'd skip any food that had it listed.
Ethylene glycol is the very toxic antifreeze one needs to keep away from pets.
Propylene glycol is also an antifreeze, is more corrosive and less toxic than the above. It's used as a food additive and in small quantities should be okay to give to your dogs. The generic ivermectin I give to my dogs is diluted with propylene glycol.