Although it is not toxic to cats, consuming large quantities of fresh catnip may hurt them by causing vomiting or diarrhea in rare cases. If a cat has eaten too much of it and become sick, limit further exposure.
A member of the mint family, catnip is uniquely enjoyed by cats because they have a special receptor for its active ingredient, nepetalactone, in their mouths. Though it is not addictive or harmful, this chemical can induce behavioral responses that range from drooling, jumping and running to sedation. However, cats that are very young, elderly or genetically predisposed tend not to respond at all.