Question of the Week

Why is chocolate toxic for dogs?

Sun, 20th Dec 2009

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Alvin Raj asked:

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. Theyíre both methyl xanthines. DogIn dogs, theobromine is very long lasting. So itís 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 itís just a fact that every species has a different metabolism.  We see differences between dogs and cats.  With certain drugs, say for example, you shouldnít give a cat paracetamol whereas dogs can tolerate paracetamol.  So itís 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.  So if a dog eats a couple of M&Ms, thatís 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 itís actually not very much. And the big problem 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.


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It's a chemical present in chocolate called theobromine that is the problem. Animals such as dogs and cats cannot metabolize theobromine effectively, in addition to having a lot less body weight than humans (although cats are less prone to eating large amounts of chocolate due to their inability to taste sweetness).

According to this page - a toxic dose is between 100-150mg per kg of body weight.

Different chocolates have different concentrations of theobromine:

Milk chocolate: 154mg/100gm
Semisweet chocolate: 528mg/100gm
Baking chocolate: 1365 mg/100gm Madidus_Scientia, Tue, 15th Dec 2009

Dogs are not really equipped to digest plants and their associated toxins. Their normal food - other animals- doesn't normally contain any alkaloids to speak of so dogs don't have the enzymes to deal with them. Similalrly cats are particularly susceptible to poisoning by phenol derivatives like aspirin. Bored chemist, Tue, 15th Dec 2009

Fascinating point BC.

But dogs aren't so sensitive to aspirin though are they? So how is that explained in the context of the above? chris, Tue, 15th Dec 2009

AFAIK theobromine is accumulative... so while a small amount of chocolate will probably be of little harm... making sure the dog doesn't get to the 'Easter hunt' eggs before the kids seems fair. JnA, Wed, 16th Dec 2009

If I catch any dog eating my choccy, it'll get a toxic size 9 steel toecap up it's ....... Don_1, Wed, 16th Dec 2009

Actually, dogs (& cats) can live on vegetarian, even vegan diets, & thrive. I will look for studies that support this (& no, I don't believe in cherry picking - the evidence really does point to dogs being able to eat a vegetarian/vegan diet with ease) if you require.

I apologise for the amount I have spoken AR/veganism recently, but this isn't about justice - it's about the science now. This is not an attempt to further veganism or ARs. glovesforfoxes, Wed, 16th Dec 2009

Notwithstanding the fact that dogs can eat a vegetarian diet (though you need to be careful) it remains that case that their normal food is other animals.

What did your comment add to the discussion apart from furthering the vegetarians' cause (whether it's a just cause or not)?
The OP asked why chocolate is toxic to dogs- there's a tacit point there that chocolate is, at least, a lot less toxic to people. Why the difference?

The difference is not because you can get a dog to eat a vegan diet. The difference is because for many generations, they didn't eat plants. Evolution therefore had no cause to equip them to deal with the toxins that plants produce.
Humans are omnivores (whether anyone likes it or not) and do need to be able to deal with plant toxins- at least to some degree so they can. The use of some tocins in small doses as drugs is a side benfit. I guess dogs could enjoy a "caffeine buzz" but they would need a much lower dose and they wouldn't get rid of it in a hurry.

For what it's worth I understand that rabbits (which really are prety near vegan) can eat atropa belladona (AKA deadly nightshade) with inpunity. Bored chemist, Wed, 16th Dec 2009

Nothing except pointing out an inaccuracy, or, at the very least, something which you could draw the wrong conclusions from. That's all I wish to do, in the same way I would say it's wrong if you said the sun revolves around the Earth.

This is a much different claim to:

That's an overly broad claim. That's all.

You can't use evidence for being unable to digest plant derived substance X as evidence for not being able to digest any plant derived substances. I'm sure you appreciate this. glovesforfoxes, Wed, 16th Dec 2009

DiscoverDave has given us the actual toxicity of chocolate, in respect of canines and BC has put a forward a perfectly reasonable reason why it should be the case, namely evolution. Dogs are carnivores, well the wild ones are. That is the OR answered.

I think the reason our domestic pet dogs now eat plant derived foods is a matter of economics rather than anything else. Pet food manufacturers tell us they provide a 'healthy balanced diet', I disagree and if mother nature (and dogs) could speak I think they would too. Cereals have been added to dog/cat food not for a 'balanced diet', but to keep down the manufacturers production cost. They add bulk. When did you last see a wolf pouring Bisto on its latest kill? Yet look into a can of dog/cat food and you will find cereals, jelly and gravy.

I could easily buy manufactured pellets for my tortoises or buy green leaves from a supermarket, but I would not dream of doing so. Instead I take 2 or 3 back breaking trips a week to pick dandelions, hawk bits, plantains, sow thistle and so on, because that is what they have evolved to eat. Who am I to question 300 million years of evolution?

Man has interfered with nature quit enough as it is, dogs/cats are carnivores, we should not try to change that. Don_1, Thu, 17th Dec 2009

I'm afraid that is all rather true. We have two terriers. A Scottie and a Cairn. Normally, butter would not melt in their mouths, but give them an inch, and they'll rip your bleedin' leg off. Geezer, Thu, 17th Dec 2009

very erudite and interesting thread.

however, I feel that this thread has gone off track in one major way. That theobromine is the culprit is beyond dispute.

However, in wondering why it is toxic to dogs and the evolutionary basis of this, you should acknowledge in my opinion that theobromine is also toxic to humans too. It is a form of a xanthine; xanthines are a particular class of chemical, and caffeine is one member of their family.

We've all heard about the dangers of caffeine overdose. Drink too much coffee and you'll either be a high as a kite or as stressed as nobody's business. It just happens that humans are susceptible too, but the dose required for the same toxic effects is much higher due to the greater mass/weight of the human compared to dog. There are also differences in how theobromines are eliminated in the bodies. I suspect dogs are unable to eliminate it as readily, but I have no proof of this. I know that human babies can't eliminate it well. Therefore, as they're smaller people as well, I am told reliably that they respond badly to chocolate.

That's enough from me obviously. I hope the rest of the thread goes well. Shibs, Thu, 17th Dec 2009

I have started a new topic about pets & animal based foods.;topicseen glovesforfoxes, Thu, 17th Dec 2009

Just as soon as someone shows me the part of a dog's DNA that codes for either the abillity to digest cellulose, or the weird multi chambered stomach or two pass systems that cattle or rabbits respectively use to digest cellulose (with, where appropriate, help form microorganisms) I will accept that I was wrong to say "Dogs are not really equipped to digest plants and their associated toxins."
Of course, if someone can show that cellulose isn't a major part of plants I will also accept that.

Until then I logically cannot accept that there was a need to post anything to correct my assertion that "Dogs are not really equipped to digest plants and their associated toxins"; it was, and remains, true.

BTW, re "you should acknowledge in my opinion that theobromine is also toxic to humans too. "
I accept your opinion, but challenge it, or at least it's relevance. There's plenty of theobromine in chocolate and I eat lots of it. Because I'm not dead  we know that theobromine isn't very toxic to humans .
To say that it's slightly toxic to humans is true but redundant. Water is slightly toxic to people. So is everything else, ask Paracelsus.
I also questiomn this assertion "It just happens that humans are susceptible too, but the dose required for the same toxic effects is much higher due to the greater mass/weight of the human compared to dog. "
It's true that most humans weigh more than most dogs but, even on a weight for weight basis, theobromine, caffeine and theophylline are, so I understand, more toxic to dogs than to humans.
Discoverdave's post supports this. Bored chemist, Fri, 18th Dec 2009

Fine by me, BC, if you're willing to redefine "toxins" & also admit that using your criteria, humans are not really equipped to digest all plants since we have no microorganisms in our gut which produce the required enzymes to break down cellulose & do not make them ourselves. glovesforfoxes, Fri, 18th Dec 2009

Lots of information about theobromine metabolites being the toxic culprits responsible for poisoning dogs that have ingested chocolate...but WHY are these methyl xanthines bad news? What do they do to the dog? How would one recognise a chocolate-intoxicated animal?

Chris chris, Sun, 20th Dec 2009

Thanks Dave; so it's chiefly potentiation of adrenergic signalling -  a bit like a caffeine OD in humans... Meera, Mon, 21st Dec 2009

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