Not all chew toys, while ubiquitous, are not completely safe. The chemicals used in the tanning process can be deadly to dogs. Rawhide from China may be even more dangerous because of lack of government oversight.
After slaughter, the skin of the cow or bull is processed to remove all the meat, leaving just the hide. Factories use a solution of harsh chemicals such as lye, lime, arsenic and formaldehyde to strip the meat from the hide. The hide is then rinsed with bleach, formed and left to dry, where it shrinks considerably.
When your dog re-hydrates the hide with his saliva, it almost doubles in size, causing gastric problems such as pain and blockage that may require surgery. This happens most often when your dog bites off small pieces of the hide that become gummy when wet so form together to make a bolus in your dog's tummy.
While chewing on a rawhide stick or bone, dogs bite off small pieces that pose a choking hazard. Intestinal obstructions usually require laxatives to help move them along -- or surgical intervention.
Food allergies are almost always caused by an aversion to the protein used in the dog's food. Hypoallergenic food diets are made with a variety of non-beef proteins. Rawhide chew toys found in most stores are made from beef. If your dog is on a hypoallergenic dog food and chews on a cow hide, you're defeating the purpose of the special food and introducing beef into his diet.