What is Catnip?
Catnip is the common name we give to this perennial herb from the mint family. It's Latin name is "Nepeta cataria". The active ingredient in catnip is called Nepetalactone. The olfactory system (or sense of smell) is the part of the brain responsible for the joyous ecstatic behavior exhibited by cats sensitive to catnip. Not all cats respond this way. Very young kittens and senior cats do not respond as strongly. Approximately 20% of cats do not respond to catnip, ever. The reaction to catnip is hereditary--some cats are genetically programmed to respond and some aren't.
The active ingredients in catnip come from the essential oil in the leaves. These become stale quickly if stored in the light or in warm temperatures. Store loose catnip in a jar or reclosable container in a cool dark cupboard for longest shelf life. If your cat has tried store bought catnip and didn't seem to care for it, it may be that your kitty is immune to catnip or, as is often the case, the catnip was stale when you bought it.
Catnip is generally quite safe. Some cats get more aggressive under it's influence-- cats are individuals too, just like people.