by Pat Gray

Created on: January 20, 2009   Last Updated: June 17, 2009

We all know that chocolate + dog = bad, but not all of us know why. We've all heard first (or second or third) hand accounts of dogs who ate copious amounts of chocolate and lived to tell the tale. Is all chocolate bad for all dogs? How much chocolate is a problem? Is it worth the risk?

Warning: Science Ahead!
Chocolate comes from the "Theobroma cacao" tree and contains theobromine, which is an alkaloid similar to theophylline (used in the past to treat asthma) and caffeine (used to treat sleepiness). Medicinally, theobromine is used as a diuretic (increases urination), vasodilator (increases blood flow) and myocardial stimulant (increases heart rate). Chocolate also contains caffeine, which is also dangerous for dogs.

Humans tolerate theobromine, and even doctors admit to the beneficial effects of chocolate. Dogs aren't so luck - they lack the enzyme that metabolizes theobromine effectively, and poisoning occurs because theobromine takes so long to leave their bloodstreams.

While in the dog's system, theobromine effects the central nervous system, heart and kidneys, causing increased heart rate and urination, vomiting, tremors and in more severe cases, seizures or heart attack. If we're talking milk chocolate, diarrhea, enteritis (swelling of the small intestine) or pancreatitis (inflammation of the pancreas) may also occur.

Warning: Math Ahead!
In general, 20 milligrams of theobromine per kilogram of body weight can cause discomfort, 45 can cause severe problems and 60 milligrams can cause seizures. A lethal dose can be as low as 100 milligrams per kilogram of body weight.

Milk chocolate contains around 65 milligrams of theobromine per ounce/baking chocolate around 440 milligrams per ounce.

In English, for a 10 pound dog, seven ounces of milk chocolate or one ounce of bakers chocolate could prove fatal. For a 50 pound dog, 35 ounces of milk chocolate or five ounces of bakers chocolate could prove fatal. These are general guidelines - for a dog with heart or kidney problems, a dangerous dose may be much lower.

So, does it hurt your dog to get an M&M as an occasional treat? Probably not, but why risk giving them a taste for something that could prove lethal?

Finally, if you suspect your dog has eaten a dangerous amount of chocolate, don't wait, contact your vet immediately. Symptoms of a fatal dose may not develop for 12 to 24 hours and early treatment may save your dog's life.

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