Hello, thank you for using PetCoach.
Allergies are extremely difficult to manage especially with a dog that is not amenable to treatment. The strongest oral sedatives available can only be gotten from your veterinarian. They cannot be purchased over the counter without a prescription. The best over the counter medication I have seen work well is called Zylkene. It is not cheap but I think it is worth a try. He would need the large dog size and would get one per day. I would give one per day and use it everyday until the bottle runs out (30 days) and if it works, great, you can keep him on it. If it doesn't work, then you can stop.
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I would also recommend a leave-on mousse instead of a shampoo. These are medicated and work very well either in between baths or for dogs that are not amenable to bathing. I would use the "CALM" when he has no lesions on his skin. If he does have lesions on the skin (hot spots, etc) then I would use the S3 PYO until the lesions are healed and then go back to the CALM. You don't have to do his whole body - just the major affected areas. You need to make sure it gets on his skin and not just on the hair. You can use it every day if necessary - you will just need to see how he is doing which will dictate how often to apply it.
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Finally, I would recommend putting him on a fatty acid supplement if he is not already on one. This helps to maintain the integrity of the skin. I like this one:
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Honestly, the best option is to take him to your vet and discuss with them a medication called Apoquel. It works great at keeping allergies under control, it is an oral tablet so typically easy to give and it will keep him from scratching and itching and having all the allergy symptoms. This way he is comfortable and you don't have to force topical treatments on him.
Hope this helps. Best wishes.