Why is chocolate toxic for dogs?

We put this question to Sorrel Langley-Hobbs from the Vet School at the University of Cambridge...

Yes. Chocolate, unfortunately, is toxic to dogs. And the reason for that is because it contains a compound called theobromine.

Theobromine and caffeine are both present in chocolate, but theobromine is the problem. They're both methylxanthines.

In dogs, theobromine is very long lasting - so it's got a very long half-life of about 18 hours - whereas in people, the half-life is only two or three hours. And people readily absorb the theobromine.

I think it's just a fact that every species has a different metabolism. We see differences between dogs and cats with certain drugs, say for example, you shouldn't give a cat paracetamol whereas dogs can tolerate paracetamol. So it's just a species difference; probably down to different enzymes that are present in the system.

So how much theobromine is toxic, you might ask yourself.? If a dog eats a couple of M&Ms, that's not going to cause any problem. The toxic levels vary from 20 mg per kilogram of theobromine to about 150 mg per kilogram of theobromine. So what does that mean in reality? Well, putting into a typical scenario, if you got a labrador and that ate a 200 gram bar of dark chocolate, that, potentially, is enough to kill your dog. So it's actually not very much.

The big problem at this time of year is someone gives you a box of chocolates, wrapped up, and you put it under the Christmas tree... and the dog eats the box of chocolates. If that happens, you certainly should call your vet as soon as possible.