Licking bald spots

Question
my cat is having a problem where he is licking himself bald in little nickel sized spots on his forearms/neck/back legs anywhere within reach.  There is no pus, they don't scab and the only time it bled was when he had one on his neck and he scratched it with his back foot.  they're just bald spots.  he licks one, it heals and then he licks a new one.  when i brought him to the vet all they did was a culture for ringworm, mites and mainge and it was all negative and he has no fleas.  they gave him a steroid shot and some medicine to heal it but that's only helpful temporarily and i don't want to have to go to the vet and pay $150 every time he licks a bald spot on himself.  do you have any ideas?  Also, my friend told me that her dog has this problem and their vet told her to use human benadryl for him but is that safe?  it is safe for her dog because she gives it to him, but is it safe for a cat?  i'm afraid to try because even if it's safe i might O.D. him.  I'm very frustrated and feel bad that i can't help him.  thank you.

Answer
If the steroid shots help, ask the vet if you can come in once every 6 weeks for just the cost of the shot.  That is what I do for my allergic cats with this problem. Benadryl may help but only if it is given orally.  The antihistmaine in cats that works the best is chlorpheniramine.