The pupils dilate for many reasons. They mainly change their size with the intensity of light: pupils become smaller when the light is bright, while in the dark, pupils dilate. If dilated pupils do not result from a normal response to low light, thus they may be caused by therapeutic medications or drugs, biological or chemical toxins and poisons as well as diseases involving the nervous system.In addition, dilated pupils may result from physiological stimuli such as fear, but in general, such reaction is not obvious and the pupil will become slightly more dilated temporarily.