While effects of the Stun Gun do not cause serious injury, they are effective and will bring the attacker an extreme amount of discomfort and pain. In a nut shell, the stun gun sends a shock wave to the nervous system and essentially over works the muscles. This causes severe pain. The impact of the shock is affected by the size of the attacker and how long you shock them for. The longer the device is held on the attacker, the greater the pain. See below for further explanation on the length of a shock and its effects.
A short blast (1/4 of a second): This will startle the attacker. It can cause minor contractions and have a repelling effect. If your attacker lets go of you, get up, get out, run, and call the police.
A moderate blast (1-4 seconds): This can cause the attacker to fall to the ground and cause mental confusion. This will deter further attack but they will be able to get back up very quickly. As soon as they begin to fall, make your break to safety and call the police.
A severe blast (5 seconds or more): This can immobilize an attacker. It will cause disorientation, loss of balance, falling to the ground, and leaving them in an unpleasant daze for several minutes. If possible, this should be your goal. Again, once the attacker has fallen to the floor, get out and call the police.