I read somewhere that cats have censors in their brain that turn on and off around catnip- thats why a cat will be flipping out over it and then it'll be like nothing and then start flipping out again.
It depends on the cat and what type of catnip. Fresh cat nip, not od really, but can cause vomiting and diarrhea in some cats. Just like us animals are all different and are affected in different ways by things. Its usually good to introduce them to a small amount at first and there is no reason they need to eat it. They will get the "high" just from the scent so if you are worried I would just rub it on a scratcher or get toys that keep it inside so they can smell it but not ingest then you have no worries
Nope. Catnip is just a form of mint (it's good for their stomachs). Humans would just as likely OD on Basil. Besides, most cats build a tolerance to catnip after fifteen minutes, and it can take about 2 hours before they will react to it again.
In my non expert opinion, I don't think so. The cats I've had that do like it, not all cats like catnip, only play with the object that has it for a very short period of time before moving onto something else, another toy or nap.
I have thought about it, but why would they sell something that they could OD on when it's in their cat treats and toys as well lol. It would not be a fun time if that would happen. But to answer the question, no they can't :)