When we met Jager, he was a hefty 25 pounds -- ridiculously overweight. It took awhile for him to slim down, but he's now a svelte 12 pounds. He's happier, much healthier, and way more active than he was originally. We got Jager when one of the vets with whom we work contacted us and said his owners had brought him to the vet clinic and asked that he be euthanized. They contended he was nippy and said they no longer wanted him. 

Maybe the reason for that was that, in addition to being in pain from carrying double the weight he should have been, he had Cushing's disease, which causes the adrenal glands to produce too much of a certain hormone, usually cortisol. This can cause a dog to be hyperalert and uncomfortable, certainly something that could cause nippiness. Jager has been on medication for Cushing's for a while now, and he's a different dog. He loves to hang out with people and other dogs, and there's not a mean bone in his body. While he will have to take his medication for the rest of his life, it's worth it because he's such a happy little guy.