Just about all dog snacks need to have some sort of preservative to keep them from going bad.
This can be all natural ones (and safe ones) made from vitamins B, C, or E (often seen on labels as tocopherol or ascorbate), plant extracts such as rosemary and/or citrus.
Or, they can be artificial ones (also referred to as synthetic) which can cause those allergic reactions, digestive upset and even poisoning with long-term consumption.
Some are even being looked at as possible cancer causing agents (some are actually used as pesticides).
Some of the worst additives include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butyl hydroquinone (TBHQ), propyl gallate, and ethoxyquin.
You’ll also want to steer clear of by-products. Often with innocent sounding terms such as ‘chicken by-product’, this is actually parts of animals that are unfit for human consumption. It can include hooves, lungs, beaks, stomach lining, and genitals.
In addition, fully legal is the fact that by-products can contain 4D livestock animal parts, which are taken from dead, dying, diseased, or disabled animals.