We never let the cat i grew up with have catnip cause she all ready had an extreme wild streak (well not with me, she was the sweetest, gentlest cat I ever had the pleasure of sharing my childhood with). But when she was 20 I did give her a toy with catnip and this cat went from barely able to walk to seeming like she had the energy of a fairly young cat.
I will sometimes get catnip toys to my current two, but my calico will sometimes get carried away with playing rough so I limit the exposure to catnip toys.
I do know my smaller cat cannot handle the cat nip flavoured cat treats. I gave her 2 small kibbles and shortly after there was a lot of cat sick. It was Pounce catnip flavoured treats. Come to think of it, both my cats get a little bloated and both will be sick with other flavours of Pounce. Sorry about straying off topic.
Chey
