According to ASPCA’s poison center, Avocado contains Persin, which is a toxin harmful to your dog. In detail, they report that Persin causes stomach upset, breathing difficulties, and can cause fluid build-up in your dog’s chest. Additionally, avocado contains fat. Although this type of fat is considered healthy, it can make your dog gain weight. If you insist on feeding your dog avocado, experts recommend you test them for toxicity before feeding them to your dog.