Catnip can cause incredible reactions when given to cats; however, not all cats are affected by catnip because the ability to detect it is inherited. Only 2/3 of cats actually respond to it.

Catnip contains a chemical known as nepetalactone, a terpene, that is thought to mimic feline sex pheromones. Cats sense it through their vomeronasal organs. When cats sense the bruised leaves or stems of catnip, they will rub in it, roll over it, paw at it, chew it, lick it, leap about, then purr loudly, growl, and meow.

This reaction only lasts for several minutes before the cat loses interest. It takes up to two hours for the cat to "reset" and then it can come back to the catnip and have the same response as before. Young kittens and older cats are less likely to have a reaction to catnip but big cats, such as tigers, seem to be extremely sensitive to it.

Cat owners do not need to worry about giving their cats catnip because there are no side effects to it. Other plants that also have this effect on cats include Valerian and plants that contain actinidine or dihydroactinidiolide (Smith, 2005).

http://encyclopedia.thefreedictionary.com/catnip