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Chocolate contains a group of chemicals called methyl-xanthine alkaloids.
The most common of these toxic substances in chocolate are theobromine and caffeine. Ingestion of these alkaloids in sufficient quantities can cause a number of serious effects in the body. They act on the gastrointestinal, cardiovascular, and neurologic systems. Gastrointestinal signs of chocolate toxicity are non-specific and include primarily vomiting and diarrhea. When the nervous system is involved, the dog may show restlessness, hyperactivity, muscle tremors, and even seizures.