Chocolate cannot give a dog heartworms, only certain mosquitos can do that.
The first signs of chocolate poisoning are vomiting and diarrhea, increased urination and nausea. These can progress to cardiac arrhythmias and seizures and, ultimately, death.
Dosage is the key - the small amount Cola got into is nothing at all to worry about. Baker's chocolate is the most dangerous chocolate because it has the highest concentration of theobromine (the dangerous compound in chocolate).
Relative theobromine content per ounce for various products is:
Milk chocolate: 44 - 60 mgs/ounce
Unsweetened baking chocolate: 450 mg/oz
Cocoa meal: 300 - 900 mg/oz
Cocoa beans: 300 - 1200 mg/oz
Hot chocolate: 13 mg/oz
The lethal dosage of theobromine in dogs is between 250 and 500 mgs/kg body weight.
Theobromine comes from the plant Theobroma cocoa and is present in chocolate, cocoa beans, cocoa bean hulls, cola and tea. It is also present in cocoa mulch, so do not use this in your gardens if your dog can get to them.
Theobromine and caffeine are in the same class of compounds:
http://www.hersheys.com/consumer/nutrition/theobromine/
Here are some more sites about chocolate/theobromine toxicity in dogs:
http://www.apogeecomgrp.com/drkevin/chocolate.html
http://www.idir.net/~wolf2dog/chocolate.htm
http://www.frii.com/~phouka/dogs/dog_choc.html
http://www.vetheart.com/choc.html
http://vetmedicine.about.com/librar....htm?once=true&
(Probably more than you ever wanted to know about theobromine!)
I still recommend avoiding chocolate since some dogs are more sensitive to it, but a small amount will not harm most Boxers. (Mine have been known to sneak a chocolate-chip cookie or two

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