The chemical compound in the plant that attracts and affects cats is called nepetalactone.
Nepetalactone is a stimulant when sniffed by a cat, producing a "high" that is described as being similar to either marijuana or LSD.
The effects last for about 10 minutes before wearing off and the cat going back to normal.
When a cat eats catnip, it acts as a sedative, but when smelled, it causes the cat to go crazy.
Cats may rub against and chew on catnip to bruise the leaves and stems, which then release more nepetalactone.
Catnip is safe for cats. If they eat a lot, they may vomit and have diarrhea, but will return to normal given time (and no more catnip).
Cats may react to the plant by rolling around, flipping over, and generally being hyperactive.
About 50 percent of cats seem to be affected by catnip, and the behavior that results varies widely between individuals, and it is believed to be an inherited sensitivity.
And if your cat does have the sensitivity, it will not emerge until your cat is several months old, young kittens are not affected by the chemicals in the plant.