chesapeake bay retriever biting herself

Question
hello,  i have a 4 year old Chesapeake bay retriever that is having some major skin problems.  in the last few months she has chewed her tail raw to the point that it was dripping blood, and she chews on her rump just above the base of her tail making it bleed.  we took her to our local vet and she gave her a hydocortizone shot and put a lamp shade on her until she heals.  that almost three months ago.  she has to wear the cone all the time because as soon as you take it off her she starts chewing, she even tries to chew with the cone on!  the local vet suggested that we may need to to try allergy testing, but my concern with that is, that her chewing has become behavioral and even with allergy shots she will continue to hurt herself, and that is assuming that the shot will help.  i am scared that she will need to be put down and am looking for a second opinion from a fresh set of eyes.  any ideas on what is happening and what we need to do?  thank you for your help.

Answer
An "allergy shot" will only last a few days.  What you are describing is typical of flea allergy.  That requires a fairly high amount of cortisone on a continual basis.  I would recommend a shot of dexamethasone in the muscle (6mg) and then dexamethasone tablets 1.5mg every other day for at least 2-3 weeks to get the dog to stop shewing.  If there is an infection on the skin, then the dog will need antibiotics as well for 2-3 weeks.