I put a table spoon on a piece of cardboard and so far it does not seem to have a negative effect. She rolls in it and it gets all over her. So that leaves very little for her to eat.
Photos of Ezzabella rolling in the nip
Here are some comments I found
A domestic cat demonstrating the effects of catnip: rolling, pawing, frisking, etc.Catnip and catmints are mainly known for, and named after, the effects they have on cats, particularly domestic cats. Approximately two thirds of cats are susceptible to the effects of catnip, as the phenomenon is hereditary.
The effects of catnip seem to change from cat to cat. I have one cat who drools & rolls on the floor, I have another one who becomes very hyperactive, a third becomes aggressive, and picks fights with the other cats when he's had catnip.
If your cat is one that likes to eat it, then you may want to watch how much your cat eats because catnip, yet not poisonous, is a toxic plant. It's not deadly, but it can cause stomach irritation if too much is consumed.
Giving a cat more than a tablespoon a week will make the effects of the catnip become less and less over time.