If you think he is biting because of the catnip, put him on a leash and walk away. If he does not stop after about 15 seconds, take him back to his cage so that you can wash your hands thoroughly. Washing your hands will prevent him from getting any more catnip scent on them than necessary which may discourage him from nipping at other cats in the future.
Catnip has a pheromone in it called “Nerolidol”, which makes kittens bite when they get too much of it in their system by licking their faces or paws. The kitten will lick himself clean if possible, but usually cannot remove all the catnip oil from its fur until he bathes again. This is why some people advocate waiting an hour for a kitten to calm down before bathing them — this allows time for the pheremones to dissipate over time without affecting behavior later when no longer bathed with fresh pheremones being applied to them [source: Pet Supermarket].
In addition, many human skin cells have compounds similar enough to be activating triplet receptors (the same ones found in cats) giving us a stronger reaction towards certain smells than our pet counterparts do [source: ScienceDaily]. In fact according to one study, chemosensory perception was higher among humans compared with domestic dogs and non-human primates suggesting that we are more sensitive toward odors related to food sources [source: ASP