Chocolate contains a chemical called theobromine, as well as caffeine, says Dr. Jerry Klein, chief veterinary officer with the American Kennel Club. “Both [ingredients] make chocolate highly toxic to dogs,” he says. “Cats may also be affected, but they generally tend to avoid sweet foods.” Dark chocolate and unsweetened, bitter chocolate are the most toxic, Ross says, because they contain the highest concentrations of the methylxanthines (theobromine and caffeine).
Signs your pet ingested chocolate can include hyperactivity, diarrhea, vomiting, elevated or abnormal heart rates, or even seizures. “Pancreatitis is also a concern when dogs eat chocolate,” Klein adds. “Chocolate ingestion can be an emergency, so pet owners should contact their veterinarian immediately if they suspect their cat or dog has consumed chocolate.”
To determine whether your dog has ingested a potentially dangerous amount of chocolate, go to petMD's Dog Chocolate Toxicity Meter.