Have been battling seasonal allergies (hay fever) in our 12 yr. old border collie/lab cross female for 7 years. They last from Aug. till frost hits in Nov. here in Canada. She came as a rescue from another family and we immediately started her on the raw BARF diet, which we have been feeding our dogs and cats for many years. Have tried the ACV in her water, (she doesn't like to drink so we squirt it down her mouth many times a day) an ACV spray externally; both were working until mid Aug. when symptoms came on full blown.
We have tried the vet route with antihistamines, that only work for about 1-2 hours and the latest one we tried, agitated her to the point she and we, were up all night. The ACV spray is working on her itchiness but needs to be administered often so while on the hunt for a tranquilizer or something that would let her (and us) sleep most of the night, I thought "what do humans use" and thought of melatonin. It is a natural hormone, esp. used for jet lag, etc. and while researching, discovered that in dogs, it is very effective for thunder fear, anxiety, and sleeping as it acts as a mild sedative, with very few side effects. So we bought some (super cheap) and it worked immediately.
So for 3 full days now, we have administered 3 mg. (1 tab) every 8 hours. Since she is elderly but fit, she does need to sleep so we are doing this schedule. For other sizes they recommend 1.5 mg. for small dogs, 3 mg. for medium and up to 6 for large over 100 lbs., every 8 hours. Hope it continues and we are still using the ACV spray on her externally to relieve the itch many times per day.