I also have a chihuahua with/had an itching problem. I went to ALLERGICPET.com and bought the "SKIN-EZE' package, an all natural product for small dogs. $84.00 and worth every penny. No visits to the Vet this last summer which is a savings in its self. And no steriods for my lttle furbaby. I gave her a capsul at night, it sedates just a bit, and she slept soundly all night. At the worst times I gave it to her morning and night. If needed I used the spray for on the spot itching. And I use the shampoo for her baths. I haven't had to use it for about 3 weeks now and she is fine. I was so worried about all the steriods for such a small little body. Be sure to read the reviews, that is what sold me and prompted me to buy the product.
Good Luck and hope it helps you as much as for us.
Linda