Dog, if used improperly, can be a big problem for your dog. While you can't hear the high-pitched squeal of the whistle, your dog has much more sensitive and powerful hearing. This means that sounds that don't bother you can be annoying or even painful for your pooch, and if you make those sounds indiscriminately, you can cause him unnecessary discomfort. Designed to be impossible for humans to hear, dog whistles aren't inherently harmful for your animal -- to an extent. Dogs are able to hear sounds of a much higher frequency than humans are, and the dog whistle creates one such sound. While a human can hear sounds around 23 kHz, dogs can hear sounds up to 45 kHz -- a significantly higher frequency.
Because dogs have much more powerful senses of hearing than humans, dog whistles that people don't even notice may be painful for dogs when abused. A loud noise blaring in your ears would cause you pain, and blowing the whistle too close to your dog or with too much force may similarly hurt him.