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Steroids are absolutely the worst thing you can give your cat on a long term basis. You really need to find out what is causing the problem with your cat's itching. My initial guess and this is true probably 80-90% of the time, is that your kitty has food allergies. Most of the time, when there is a food allergy, the problem is corn or corn gluten which is the main ingredient in many commercial foods.
I would start by getting your cat on a corn-free food such as Innova, Wellness, Felidae or California Natural. When you put her on the new food, add just a little of the new food to the existing food she eats and each day add a little more new and a little less old until she is eating all new food in about a week to 10 days. You should then start watching to see if the itching slows down or stops.
If it still continues, I would get a product from the pet supply called Microtek Spray. You spray this on their coats and it helps greatly with itching and can cure bacterial skin problems as well as fungal problems and body mites. You can also order it online at www.revivalanimal.com - it is very soothing and it should help her skin a great deal. I'm assuming your vet has checked thoroughly to ensure there are no fleas. You might try using some Revolution on your cat - even if there are no fleas, it will also help by killing any body mites that may exist.
Let me know if putting her on the corn-free food helps.