I asked my vet the same question last year because they were going nuts with it. He said there are no known side effects. I just restricted access because I wanted it to last a little while.
Katharina

Think of it as medicine with a punch. Cats eat greens to help either pass through or regurgitate undigestibles such as hair. Catnip serves the same purpose and gives them a buzz too.

I do restrict access with mine too though. If they're stoned all the time I'd think that they might get bored with it.

I have some catnip seeds to sow. The pack does say that not all cats like all varieties of catnip. Knowing my luck my cat won't like the variety I have bought. But that says it all about my cat. He won't play with the toys I buy him but he will play with my laptop charger. He won't poo in his dirtbox but he will poo in my flowerbeds. He won't use the scratching post I bought him, instead he has the back gate nearly worn through. Despite this he is my cat, my master, and I love him.