Chocolate is dangerous because of its theobromine and caffeine (methylxanthine) content. Signs appear at around 20 mg/kg, severe signs at 40-50 mg/kg, and seizures can occur above 60 mg/kg. Baking chocolate (up to 450 mg/oz) is the most dangerous, followed by dark chocolate (130-450 mg/oz) and milk chocolate (44-58 mg/oz) — but no amount is truly safe. Signs typically appear within 6-12 hours of ingestion, and cats and birds are similarly susceptible.