You can grow your own, you can buy packaged dried catnip or you can buy catnip-infused toys. If you grow your own, don't grow it outdoors if other cats can get into your yard or you'll be the most popular house on the block. If you grow your own, you can dry the catnip by hanging it in bunches upside down in a dry, dark place. After the herb has dried, strip off the leaves and blossoms (don't use the stems), and store in a tightly sealed container. Don't crush the leaves until you're ready to use the catnip because you don't want to waste it by releasing the oil prematurely.
If you buy packaged catnip, look for brands that use only leaves and blossoms. Catnip that contains a lot of stems is of lesser quality. If the catnip is in a bag, repackage it in a tightly sealed container. When you're ready to offer the catnip to your cat, rub some between your hands to release the oils. You can offer the catnip loose or put some in a sock and tie a knot at the end. I also keep a few furry toy mice containing catnip. Rub some catnip on scratching posts periodically as well, if you're in the middle of training a cat to use the post.
Don't buy catnip-filled toys unless you know the quality of the manufacturer. Some catnip toys are filled with low-quality catnip, and some may not even be filled with true catnip at all.
In addition to offering catnip during times of stress or after a particular event, you can use it on a maintenance basis but not more than once a week.
Finally, keep catnip in a place the cats can't access. You'd be surprised at how determined a cat can be. I've seen many plastic containers get demolished by a cat when the owner inadvertently left the catnip out on the kitchen counter.
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Reprinted from Cat vs. Cat: Keeping Peace When You Have More Than One Cat by Pam Johnson-Bennett © 2004. Permission granted by Penguin Putnam.