What's the reason that dark chocolate is poisonous to dogs but not to humans?

Chocolate toxicity in animals is caused by a chemical caused theobromine, which is naturally produced by cacao plant (and some plants that are used in tea). The more cacao (i.e. the darker the chocolate), the more theobromine will be present. It messes with the chemical signals that the body's cells use to communicate, causing dilation of blood vessels, an increase in heart rate, and some effects on nerve cells.

It has these effects in all mammals. It's just that dogs, cats, and some other animals, for one reason or another, have metabolisms that don't break down theobromine very quickly. That means that it stays in their blood stream for longer, causing more severe effects. It's also due to the fact that animals generally have much lower body weight than humans, so a smaller amount of theobromine can cause more damage. It CAN be toxic to humans as well... but most healthy people would probably get sick from eating so much chocolate before they started noticing any symptoms. Some people, like the elderly - who's metabolism isn't in that great shape anyways - can start to be effected by theobromine without having to eat a ridiculous amount of chocolate.

As with all things in biology, the poison is in the dose. That means the greater the dose, the greater the effect. While it's toxic in large doses, smaller doses can actually have medicinal use in treating asthma and high blood pressure, among other things. Of course, this would involve a prescription for pharmaceutical theobromine... trying to treat these problems by just eating chocolate probably wouldn't work very well, as much as I'd love for my doctor to tell me to eat more chocolate.

The toxic component of chocolate is theobromine. Humans easily metabolize theobromine, but dogs process it much more slowly, allowing it to build up to toxic levels in their system.

A large dog can consume more chocolate than a small dog before suffering ill effects.

A small amount of chocolate will probably only give your dog an upset stomach with vomiting or diarrhea.

With large amounts, theobromine can produce muscle tremors, seizures, an irregular heartbeat, internal bleeding or a heart attack. The onset of theobromine poisoning is usually marked by severe hyperactivity.

Chocolate contains theobromine. Humans easily metabolize theobromine, but dogs process it much more slowly, allowing it to build up to toxic levels in their system.

It's toxic to dogs because their systems can't process it. Just like you can't lick your own poop without getting sick. Our systems are different.

God wants you to give chocolate to your girlfriend and not your dog.