Since the best catnip can work wonders on your cat, and help to keep them calm and happy, there's no reason why you shouldn't use catnip if your cat is in heat. Just work with your cat's personality, and don't give them any more catnip if your kitty becomes anxious or more excitable. While normal play, some chewing and general fun are expected, cat owners should be aware if their cat becomes aggressive or unhappy. Thereby taking away the catnip, if necessary.