Cats that are catnip responders can have a real range of reaction, but I've never heard of a cat being allergic. To even respond, kittens have to begun to be sexually mature, and have inherited the ability to respond. I have a good little catnip faq on my site..
Catnip is often jokingly referenced as marijuana for kitties, but it has a much less intense effect. I've never heard of it being harmful to kitties, and if it were I doubt we'd have pet stores flooded with catnip products.