Okay, since I did SO WELL at ID epi, I’ll have a go at veterinary toxicology, because, y’know, data.
Do not let your dog eat chocolate. Chocolate can kill your dog. I think everybody who owns a dog knows this (well, I wish everybody did) but I’ve heard a lot of contradictory and counterintuitive things about exactly how much chocolate not to let your dog eat, and why. Yesterday, while I was (ironically) observing on a toxicology service, my new puppy got into a food bin and, along with a bag of noodles and some dog treats, ate an enormous bar of Dagoba baking chocolate. This gave me the excuse I needed to look into this chocolate toxicity- and don’t worry, the dog is fine, albeit stupid.
Methylxanthines Is How Smart People Wake Up
Caffeine is a member of a chemical family known as the methylxanthines, which are produced by plants and have similar effects. Even if you’re Mormon and have never had a cup of coffee, you probably understand through mass culture what methylxanthines do- methylxanthines improve alertness, blood flow, cause jitters, aggravate anxiety, relieve migraines, etc. As with many families of plant allelochemicals, methylxanthines come in a number of different variants that occur in different concentrations in different plants. While you will sometimes hear that coffee (Coffea arabica) contains caffeine, tea (Camellia sinensis) contains theophylline, and cocoa (Theobroma cacao) contains theobromine, the truth is that in fact coffee, tea, and even chocolate contain measurable levels of all three, as do other plants such as Yerba Mate and Yaupon Holly (possibly the source of cassina).
However, the three chemicals are somewhat different, and while all are more toxic to dogs than to humans, theobromine (which predominates in chocolate) has a longer half-life and is harder for the dog’s body to get rid of. Also, we stupid humans leave theobromine-rich tasty nuggets lying around, while coffee and tea generally come in forms less appealing to dogs.
Generally, the toxicity of a substance kicks in at a given concentration, or how much theobromine there is per unit dog. To measure concentration, we have a made-up statistic called “volume of distribution” which is how much imaginary dog there is to dissolve your theobromine in. This is because some substances act as if they were in a bigger or smaller container- if a drug is lipophilic (goes mainly into fat) very little of it will be in the blood, and it will look like we dissolved the drug in a very, very large dog. Other stuff, like injectable albumin, pretty much never leaves the bloodstream, so every part of the dog other than the blood is irrelevant. For albumin, the “volume of distribution” looks like a teeny tiny dog, compared to the actual animal. I don’t actually know this information for chocolate and dogs and I don’t much care. The critical issue here is the dose-per-unit-weight, which takes the toxic concentration and the volume of distribution into account.
The symptoms of theobromine overdose in dogs are pretty much the same as a lethal caffeine overdose in humans. For dogs, the low toxic dose is about 20mg/kg, which causes “restlessness and vomiting.” That means that for my 20kg dog (44lbs- c’mon people, learn the metric system already) 400mg of theobromine is enough to cause problems. Cardiotoxic problems kick in around 40-50mg/kg, (or 800mg-1g for my dog), and seizures around 60mg/kg (1.2g of theobromine.) The numbers are about the same for caffeine. The LD50- the “50% lethal dose” where half of all individuals die, is 100-500mg/kg (for which my dog would have had to eat 2-10g of theobromine). That doesn’t mean that lower doses are safe, just that if you want your dog’s life to rest on a coin flip, that’s how much they would eat.
Now, these are the numbers for theobromine itself. Theobromine concentrations vary in different types of chocolate. You can use pet MD’s chocolate toxicity meter which gives you six options or you can look at Hershey’s ingredients page and get more specific information. Hershey’s makes Dagoba, but doesn’t list data for their unsweetened baking bar. However, we can do some math.
Dark chocolate is made up of cocoa butter, cocoa powder, and sugar. When you see “75% cocoa” on a bar it means that the sugar makes up no more than one quarter of the chocolate bar. When you see “100% cacao” (not cocoa) it means the manufacturer is having fun with you, since both cocoa butter and cocoa powder come from cacao. Well, I guess “100% cacao” means no sugar, but so does “unsweetened” so it isn’t exactly helpful. Looking on the nutritional info, the Dagoba Baking Bar contains 15g of fat in every 28g serving, meaning that the other 13g (46%) is cocoa powder (where the theobromine is). Hershey’s claims that the cocoa powder in its Dagoba line contains 100mg of theobromine for every 5g serving.
The baking bar weighed 170g when I bought it, but I’d used about 60g already. That left 110g of chocolate, of which 46% was cocoa powder equivalent, or about 50g. 50g is ten 5g servings, so it contained about a gram of theobromine which, divided by my poor puppy’s body mass, comes to a dose of 50mg/kg, well into the cardiotoxicity range. I mentioned that the dog was okay- this was a combination of rapid-response induced puking and later finding most of the bar tucked behind the sink. We think he ate a bunch of crumbs off the wrapper, which was well chewed, but that’s it. Dogs don’t like raw unsweetened chocolate any more than humans do, it appears!
Also, we can run the math backwards. A borderline “safe” dose of theobromine (20mg/kg) would be 400mg theobromine, 20g of cocoa powder, or 43g of chocolate. He could have eaten one quarter of the bar “safely.”
And Half Life
Remember I said that the critical factor was concentration. When calculating a toxic dose, you assume immediate total absorption (unrealistic, or why puke your dog?) and no further metabolism. You can do it the hard way with differential equations if you want, but the goal here is to determine if the dog needs to get charcoaled at the vet, so precision is less important. However, the concentration drops with time as your dog recovers- this is important as one of the myths I hear is that dogs never recover, or else you calculate the toxic dose based on how much chocolate the dog has eaten in the past year, or its entire life.
Folks, the half-life of theobromine in a dog is 17.5 hours.
That means that lets say the dog ate a quarter bar of chocolate. We’ve established this would be about a 20mg/kg dose. Then, he eats another quarter bar- now we’re up to 40mg/kg and we worry. Lets say instead that he waits eighteen hours (round numbers!) between quarter-chocolate-bars. His effective dose at this point is 30mg/kg, because the first dose, after a half-life, is cut in half. You can run a series if you want- a day (24h) is 1.37 half-lives, so each morning the dog has about 39% of the theobromine it ate the previous morning, assuming no intervening doses. That means a dog could eat 25g of chocolate every morning and never have an effective dose higher than 20mg/kg. Don’t try this at home, there’s all kinds of other factors. Still, it should be a bit of a relief.