Chocolate contains theobromine that is toxic to dogs in sufficient quantities.

This is a xanthine compound in the same family of caffeine, and Toxic Levels  theophylline.  

The good news is that it takes, on average, a fairly large amount of theobromine 100-150 mg/kg

tcause a toxic reaction. Although there are variables to consider like the individual sensitivity,

animal size and chocolate concentration. On average, Milk chocolate contains 44 mg

of  theobromine per oz. Semisweet chocolate contains 150mg/oz. Baker's chocolate 390mg/oz.

Using a dose of 100 mg/kg as the toxic dose it comes out roughly as:

1 ounce per 1 pound of body weight for Milk chocolate

1 ounce per 3 pounds of body weight for Semisweet chocolate

1 ounce per 9 pounds of body weight for Baker's chocolate.

So, for example, 2 oz. of Baker's chocolate can cause great risk to an 15 lb. dog.

Yet, 2 oz. of Milk chocolate usually will only cause digestive problems.

Clinical Signs:

Xanthines affect the nervous system, cardiovascular system and peripheral nerves.

It has a diuretic effect as well. Clinical signs:

Hyper excitability, Hyper irritability, Increased heart rate, Restlessness,

Increased urination, Muscle tremors, Vomiting and Diarrhea.