I am not a fan of "prescription diets" long term, instead I want to know the cause and eliminate it.
First, if you haven't already done so, I would have the vet do a skin scraping to see if there are any answers there.
Next take a look around and see if anything changed that would coincide with the beginning of the excessive scratching....new carpet or carpet cleaned, new dog beds, different detergent used in laundry, new cleaning product for floors, any lawn products, you get the idea.
Did you start buying different dog biscuits, chewies etc.
Although environmental allergies are more common then food allergies, food allergies are certainly a possibility. Eliminating all grains from my pupper's diet cleared up my itchy Lucy.