The toxic substance in chocolate is theobromine, which is a methylxanthine related to caffeine. Theobromine toxicity leads to vomiting, diarrhea, high heart rate progressing to abnormal heart rhythm, hyperactivity, increased drinking and urination, seizures, and possibly even death. The toxic effects are dose-dependent, meaning they vary depending on the amount of theobromine in the consumed product, the amount consumed, and the size of the animal.

Different chocolate items contain varying amounts of actual cocoa, and this affects the amount of theobromine contained in these products. Unsweetened baking chocolate contains the highest amount of theobromine, 390-450 mg per ounce, followed by dark chocolate, 150-200 mg per ounce, and then milk chocolate, 44-50 mg/ounce. White chocolate has a negligible amount of theobromine in it, but it often has higher fat and sugar levels, which can still lead to stomach upset and pancreatitis.

The lethal dose for a dog or cat is based on body weight. Mild signs, such as diarrhea, develop at 9 mg per pound of animal, with severe signs, including death, developing with doses at 18 mg per pound or higher.

Here is an example of the amount of chocolate (in ounces) a 10-pound dog or cat would need to eat to see toxic effects:

As indicated, a very small amount of chocolate can be toxic. If your pet has eaten any chocolate, call your veterinarian immediately. There is no specific antidote to this toxin and it takes several days for theobromine to be excreted from the body, so hospitalization and supportive care may be necessary. Your veterinarian may recommend any or all of the following treatments, depending how much chocolate has been ingested and the signs of toxicity your pet is exhibiting:

1. Induction of vomiting to remove remaining chocolate from the stomach
2. Activated charcoal by mouth to bind and prevent further absorption of theobromine from the gastrointestinal tract
3. Intravenous fluids to prevent dehydration, replenish electrolytes, and promote flushing of the toxin
4. Anti-seizure medication
5. Anti-emetics for nausea
6. Cardiac support for increased or abnormal heart rates.

If your veterinarian is unavailable, call your local veterinarian emergency center, or the ASPCA Poison Control Hotline at (888) 426-4435. This phone number is toll-free, but there is a fee for their consultation.