Though we may call catnip "kitty crack," the herb is non-addictive and isn't even a drug (so it's perfectly safe to give to your kitty, big or small). But how does it work? And why doesn't it have any effect on humans?
Catnip comes from plants of the Nepeta genus. These plants are a type of mint and produce a host of volatile oils and other chemicals. To us, they just smell a little sweet, but most cats have a different reaction. They roll around, rub their heads and bodies on whatever you've stuffed with the herb, and often act as if they've been smoking some kind of illegal substance. Veterinarian Ramona Turner explained how catnip elicits these reactions a few years ago in Scientific American:
Nepetalactone, one of catnip's volatile oils, enters the cat's nasal tissue, where it is believed to bind to protein receptors that stimulate sensory neurons. These cells, in turn, provoke a response in neurons in the olfactory bulb, which project to several brain regions including the amygdala (two neuronal clusters in the midbrain that mediate emotional responses to stimuli) and the hypothalamus, the brain's "master gland" that plays a role in regulating everything from hunger to emotions.
The amygdala integrates the information flow from the olfactory bulb cells and projects to areas governing behavior responses. The hypothalamus regulates neuroendocrine responses through the pituitary gland, creating a "sexual response." That is, the cat essentially reacts to an artificial cat pheromone.
This reaction lasts for about 5 to 15 minutes and then a cat is immune for an hour or so. Kitties don't react to the stuff until they're about six months old, when they reach sexual maturity. And not all cats are affected—sensitivity to catnip is an inherited trait, and only about 70 to 80 percent of house cats will react. (I couldn't find statistics for big cat species, but we can see in the video above, from Big Cat Rescue, that it works on at least some individuals.)
Humans don't react in the same way because our brains are different. In us, nepetalactone acts more like valepotriates, the compounds in the herb valerian that are a mild sedative in most people. So if you can't sleep, you can try drinking catnip tea, if you can keep from laughing at your kitty rolling crazily across the floor.