Not every cat is sensitive to catnip which is an herb.
Only about half of all cats are genetically disposed to respond to the active oil in catnip. It is not certain what part of the brain is stimulated by this ingredient, but it is not harmful and can be used to help increase their use of items like scratching posts. Found in many cat treats it can encourage play. The aroma of catnip is thought to be quite pleasurable to cats.