If you have a dog you've probably heard that chocolate will make your pet sick, and that also applies to other pets like cats, rats, and mice. but that just doesn't seem fair.

Why do we get to go to town on a giant bar of dark chocolate but Fido shouldn't even have one bite? It all has to do with a molecule called theobromine which is made up of carbon, hydrogen, nitrogen and oxygen atoms.

And along with its partner caffeine, is one of the many reasons humans have loved chocolate for thousands of years. Both molecules are dangerous for dogs but chocolate contains a lot more theobromine than it does caffeine so it's the molecule to watch out for. Like caffeine, theobromine is a type of alkaloid, which is a huge class of molecules that generally contain rings with at least one nitrogen atom in them, and alkaloids often have physiological effects on humans and other animals.

Also like caffeine, theobromine makes our heart pump faster, our blood vessels dilate, and some of our muscles get more energy, which sounds great! But, too much of it can make our hearts pump too quickly and our muscles contract uncontrollably, eventually leading to nausea, convulsions, heart attack, and even death. Luckily for our taste buds, we humans process theobromine pretty quickly, so that's not something you have to worry about.

It rarely sticks around long enough to cause any harm. But, our pets aren't so lucky. They process theobromine a lot more slowly, so it can easily build up and cause those dangerous effects.

It's hard to know exactly why we evolved to be better at digesting theobromine, but it might be because alkaloids mostly come from plants, and our ancestors ate a lot more plants than early cats or dogs did. Whatever the reason, the same amount of theobromine stays in pets' bodies much longer, giving it more time to pile up and cause harm. On top of that, most pets are a lot lighter than humans, so it really doesn't take much chocolate to make them sick.

Cats don't often get poisoned from chocolate because they can't taste sweet things, so they're generally not too interested in it, But, dogs sure are. Your average adult human would need to eat about 8 kilograms of dark chocolate to get a lethal dose of theobromine. But, a medium-sized dog would only need to eat about one kilogram, and a house cat would only need a tenth of a kilogram.

Sweeter chocolates have less theobromine, so the lethal dose is higher; about 5 kilograms of milk chocolate for dogs and about a third of a kilogram for cats, but they'd be sick long before eating that much. Meanwhile, you and I will have to eat about half of our body weight in milk chocolate for a lethal dose, which might sound wonderful, but I don't want to see any of you writing "challenge accepted" in the comments, because you'd be very sick way before finishing that much chocolate. Among other things, I'm trying very hard not to picture what that would do to your digestive system But the next time to indulge in a chocolate bar, or three, just keep it to yourself, no matter how long your dog gives you those adorable puppy eyes.

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