Catnip is not safe for ALL cats. Young kittens and elderly or ill cats may not respond well to catnip and could have adverse reactions. When using catnip try a small amount of a high quality catnip, (either loose or in a toy) and be there to watch your cat. If your cat reacts in an over stimulated or agressive way, then discontinue catnip use. If on the other hand, they react in a silly and playful manner, then offering a small amount of catnip once or twice a week as a special treat is fine to do.
Special thanks to: Dr. Linda Dupont-Catzablanca Cat Clinic and Hospital