If you've never exposed a particular cat to catnip, it's a good idea to do it without the other cats at first. Some males may cross the line from play to aggression while under the influence of catnip. You'd want to know that before offering catnip to the whole group together.
• How to use catnip
Reprinted from Cat vs. Cat: Keeping Peace When You Have More Than One Cat by Pam Johnson-Bennett © 2004. Permission granted by Penguin Putnam.