According to Pets Web MD, sugar is not toxic to dogs. However, it is bad for them in large quantities, having the potential to cause the same issues it does in humans.
Sugar, in large doses, can cause a sugar high that temporarily alters the normal behavior of a dog. Symptoms of a sugar high include hyperactivity and a lack of concentration, according to The Dog Daily, followed by a low, which causes lethargy, irritability and depression. Pets Web MD and The Dog Daily describe how the over-consumption of sugar can lead to obesity, dental problems and diabetes in dogs, much the same as in humans. Sugar, in moderation, is safe for dogs, but excessive consumption is very unhealthy.