A gentle leader is a basic training tool. They are neither good nor bad. What matters is how you use them. Consequently, they can be an effective training tool for getting your dog more time walking before he or she is fully trained with a leash. Using a gentle leader too much, however, may result in your dog never walking properly while wearing a regular collar.
Furthermore, if you pull too hard on the gentle leader, it may snap your dog’s neck around too quickly. Injuries shouldn’t occur, however, as long as you use it properly.