The clinical signs of chocolate toxicity can include vomiting, diarrhea, hyperactivity, restlessness, excessive urination, muscle tremors, tachycardia, bradycardia, arrhythmias, hyperthermia, seizures, coma and possibly death. Vomiting and diarrhea can occur 2 to 4 hours after intake. Advanced signs (seizures, heart failure, coma and death) can occur 12 to 36 hours after intake.
Information provided by Barrie Arco, Patterson Veterinary
Reference: Michigan Veterinary Specialists
Chocolate toxicity (poisoning) is caused by excessive intake of the methylxanthine alkaloids found in chocolate, coffee, tea and caffeinated sodas. The active ingredient in chocolate is theobromine. The poisoning affects many organ systems and animals of all ages are susceptible.
Poisoning is common in dogs because of their habit of rapid consumption, particularly puppies and young dogs as they may be more likely to ingest large amounts of unusual foods. Although dogs are the most susceptible, the toxin has been known to affect or kill cats, birds, rodents and reptiles as well.
Dogs can smell the tasty chocolate through packaging and wrapping, even if you can't.
Toxic dosages for dogs:
Below is a list of potentially dangerous doses of chocolate. Please note that these are approximate amounts only. Every animal has varying levels of sensitivity to theobromine. Some brands of chocolate have more caffeine than others (example: Nestlé's milk chocolate has 19mg theobromine per oz vs. Hershey's milk chocolate has 8mg theobromine per oz).
A "regular" sized chocolate bar weight about 1 & 1/2 ounces. A "regular bag of chocolate chips weighs about 12 ounces and has about 2 cups of chocolate chips.