The American Veterinary Medical Association, or AVMA, states that catnip is nonaddictive, nontoxic and safe for cats to sniff and ingest. However, catnip has been linked to seizures in rats and is a uterine stimulate, so it shouldn't be given to pregnant cats or cats with a history of seizures.
WebMD states that one in two cats are susceptible to catnip, determined primarily by genetics. The oil in catnip contains the chemical nepetalactone which affects cats differently. Either they become calm and mellow or aggressively playful. The initial effects last about 10 minutes, after which it takes at least two hours before a cat can be affected by catnip again. Too much exposure can cause a cat to develop a tolerance and lose interest in catnip.
The AVMA recommends giving cats catnip to help stimulate exercise or engage interest in a new toy, bed or scratching post. Catnip can also help ease stress or settle the nerves. WebMD suggests putting small amounts of catnip into a toy or sock to entertain an indoor cat.
If cats show signs of intolerance or digestive upset, the AVMA cautions owners to discontinue use and allow them to recover. Common signs include diarrhea and nausea.