A couple of months ago, my big white cat Storm began getting these really nasty little arrowpoint-shaped scabs on his back. They obviously itched and bothered him because he was always licking them, and in fact had licked one quarter-sized spot raw. When I realized that they were rapidly spreading and not clearing up, I took him to the vet. She told me that there were several possible causes, but that it was most likely some kind of allergic reaction and recommended a steroid shot. She said that if it was effective we could expect it to keep working for three or four months.
Within a couple of days Storm's skin began to clear up and within three or four days he was scab-free. A few weeks later I noticed it starting to come back in a couple of places, but then it was gone again almost immediately, so apparently the stuff is still in his system and working.