Is your dog losing weight, almost like he’s on a diet?  You’ll need to take him to the veterinarian. There are a variety of possible reasons for weight loss. Diet, parasites, diabetes, cancer and heart disease are some causes just to name a few. To make a diagnosis, your veterinarian will ask you about other symptoms such as diarrhea and vomiting. Take note of any changes in your dogs behavior.  You may want to consider bringing a stool sample to your veterinarian.  The veterinarian will examine your dog and run a series of tests such as x rays, blood tests or a urine analysis. Then he will make an official diagnosis. Some diet changes and pet medication is all you may need.