Anyone who owns both a dog and a cat is aware of his canine friend's inclination to sneak an occasional "snack" from the cat's litter box. This very unappealing habit, called coprophagia, is normal behavior -- much to the dismay of many pet owners. Scavenging is a basic canine trait, handed down from the dog's wolf ancestors. Cat feces have high fat content, which is appealing to dogs. Though the behavior is common in dogs, you can take steps to prevent it. The main one is to simply remove the opportunity.
Place your cat's litter box well out of your dog's reach. Placing it behind a large piece of furniture or in a small area such as a closet will make it inaccessible for most dogs. If you have very small dogs, try placing the litter box in an upstairs or downstairs room, where your dog is less likely to explore.
Elevate the litter box out of the dog's reach. The cat will have no trouble reaching it. If you have small dogs, placing the litter box on stacked wooden boards is a good option. The number and thickness of the boards needed will depend on the height of your small dog. Placing your cat's litter box on a piece of furniture such as a stool or bench is also an option, but make sure your cat is able to get in and out of the elevated litter box without tipping it over and knocking it down.
Build a barrier around the litter box. You can get creative with this option, using materials you can find at a home-improvement store such as wallboard or fiberglass bathroom wall coverings. For example, you can nail wallboard together and place the "box" around the litter box with an opening that is just large enough for your cat to climb in and out.
Try installing a cat door in a door to the room where your cat's litter box is stationed. Cat doors are similar to dog doors, but are small enough to prevent all but the smallest dogs from going through. Once you've installed the cat door, keep the room door closed. While this is ideal for keeping most dogs away from a specific area, you must make sure that your cat can operate the cat door.
Use a covered litter box. Many dogs will not put their heads in the opening. For added protection, you can turn the litter box so that the opening faces a wall to deter your dog from trying to get inside.
Clean your cat's litter box frequently. Being extra diligent about scooping the litter will reduce the number of "snacks" your dog steals.
Visit your veterinarian if the problem persists. Some dogs may respond to medications.