Chocolate is toxic to dogs and a number of other animals because it contains alkaloid chemicals called methylxanthines - namely, theobromine (3,7-dimethylxanthine) and caffeine (1,3,7-trimethylxanthine). Both of these stimulate the nervous and cardiovascular systems. It’s an effect that humans seek out, and we can get away with it because we metabolize the chemicals relatively quickly. Other animals process them more slowly, so the effects are more pronounced.

If a dog eats too much theobromine and caffeine, they’ll start to show a number of symptoms, including diarrhea, vomiting, muscle spasms, excessive panting, hyperactive behavior, seizures and dehydration. They may become hyperthermic, go into respiratory failure or experience cardiac arrhythmia, all of which can cause death.

So, how much chocolate is too much for a dog? Depends on the size of the dog, and the kind of chocolate. The amount of methylxanthines in chocolate varies among different chocolate products and brands. In general, though, dry cocoa powder has the most, with around 800 milligrams per ounce, according to the Merck Veterinary Manual. Unsweetened baker’s chocolate (~450 mg/oz), semisweet and sweet dark chocolate (~150-160 mg/oz) and milk chocolate (~64 mg/oz) follow.

Based on their experience and research, the ASPCA Animal Poison Control Center considers 100 to 200 milligrams of methylxanthines per kilogram of dog to be a lethal dose. Mild symptoms can happen with doses as small as 20 mg/kg and severe symptoms, including seizures, can happen at 40-60 mg/kg. Given those numbers, as little as four ounces of dark chocolate could cause problems for a average-sized, 60lb Labrador Retriever, America’s most popular breed. How much chocolate can your dog handle before trouble starts? National Geographic has a handy calculator to figure out the amount based on your dog’s weight and the type of chocolate.

If a dog does eat a toxic dose of chocolate, there’s not much that can be done for it outside of a vet’s office if the methylxanthines make it into the dog’s bloodstream and start circulating through the body. In her book, Help!: The Quick Guide to First Aid for Your Dog, veterinarian Michelle Bamberger recommends slowing or stopping this process by the body by trying to make the dog vomit. Don’t try sticking your fingers down its throat, though. Instead, feed it a small dose (a teaspoon) of hydrogen peroxide or table salt. Your vet can handle things from there, and treatment usually involves giving the dog activated charcoal to bind to the toxins and using intravenous fluid therapy to flush them out.

Methylxanthines in chocolate are toxic for other animals, too. Cats are especially susceptible because of their small size. Luckily for them, cats lack the taste receptors that pick up “sweet” tastes, and rarely have much motivation for eating more than a nibble or two of chocolate. Both horses and humans are less susceptible to chocolate toxicity thanks to their size and faster metabolization of the chemicals. Methylxanthine poisoning can still happen to people who consume large amounts of chocolate or coffee in a small timeframe, though, and the amount of caffeine in a strong cup of coffee is enough to cause symptoms in a small child.