Avocados contain a toxin called persin, which is mildly poisonous dogs and deadly for other animals. Avocados can cause intestinal trouble for dogs, and result in vomiting, diarrhea, and lack of stool production. The biggest risk from avocados is that they can cause obstruction if swallowed whole, and the seeds are large enough to block the intestinal tract of smaller dogs. Persin is also deadly to birds and large ruminants such as cattle, and can result in organ failure and death.