Catnip is always a good source of entertainment for most cats.
It's a natural plant that is related to the mint family of plants. There is a reaction that takes place in a cat's nervous system that causes a form of high ranging from mild euphoria to hallucination as compared to LSD in humans.
Most cats over the age of 6 months experience the reaction to catnip. Most cats will want to roll in the catnip whereas others may want to eat it. 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.
There is also a remote chance of an allergic reaction, which is extremely rare but can occur. If your cat experiences seizures or becomes unconscious after exposure to catnip, discontinue use, and contact your vet immediately. The best way to allow your cat to enjoy catnip is to place it inside a sock and tie it or sew it inside the pouch so the aroma passes through the material yet limits consumption. Too much catnip can lead to vomiting and diarrhea.
As mentioned earlier some cats get a reaction that is similar to LSD and can cause your otherwise gentle cat into an angry frightened hostile animal. The once gentle animal will then become violent and may begin fighting with other animals in the house or even attack people. If this should ever occur, try your best to contain the cat and then call your vet. Your cat may or may not need medical attention but that is for you and your vet to decide.
Catnip can be a safe treat for your cat but introduce it slowly at first to make sure there are no allergic reactions and also to make sure your cat doesn't experience a bad hallucinogenic experience.