Why does my dog bite his back?

The two most common reasons are either fleas or allergies. If a dog has fleas, they tend to hang out on the back or just above the tail base. A flea comb taken down the back, focusing just above the tail, is the best way to check for fleas. Also, make sure your dog is on year-round, prescription-strength flea prevention, especially if you're in a warmer state where fleas are active year round. If a dog is sensitive to flea bites (Not all dogs are. Some don't even notice flea bites!), only one or two fleas can set off a reaction. Over-the-counter flea preventions may indicate that they have the same active ingredient as those you purchase from your vet, but many times it's the inactive ingredients that assist the active ones in doing their job and can make a world of difference in their effectiveness. These ingredients tend to be the ones that are highly-regulated and therefore prescription strength. If your dog is free of fleas, addressing possible allergies would be the next step. Your vet can provide more information on what the appropriate steps to take would be for testing or treatment trials, based on your dog's environment and location.