Active wrinkles or creases develop from making the same facial expression thousands of times. These lines are called lines of "facial expression" and are typically located at the outer eyelid area (crow’s feet) and in the forehead (frown lines). BOTOX® works by temporarily paralyzing the muscle groups that cause these lines to develop. It is best used to prevent the formation of expression lines before they become present at rest. To minimize treatment discomfort, BOTOX® injections are performed with the smallest needle possible. Benefits begin about four days after treatment and last an average of four months.