Some dogs suffer from allergies because of the environment they live in, so chemicals we use in the house/garden to clean/wash/air fresheners etc, food you feed and medications routinely given all impact on their immune system and weaken it....... it is any animals immune system ( including humans) that keeps it healthy and fights minor illnesses.
So what has your vet said your is the dog allergic to? You can have tests to check instead of using medication to cover up the 'allergy' and/ or you can also help by feeding a good quality food suitable for dogs to eat which is not full of ingredients unsuitable for dogs.....eg raw and bone instead of any commercial kibble( ALL kibbles have some 'fillers' in them which are not a suitable product to feed dogs), instead of over bathing , daily grooming which simulates the skin as well as cleans off dead coat and dander, stop using chemical cleaners in your home or in/on the dog.
100 % pure Neem oil use as a 'finish' on the coat( put a thumb nail amount on your hands and rub all over the dogs coat/down the legs/tail) and 100% pure Neem bark/leaf powder sprinkle daily on its good quality food( 1 teasp daily) this is a natural product used for thousands of years for humans and has been extensively used for decades in the equine world, I have used it for the last 3 decades with my dogs and it stops all parasites so no need for tick/fleaing/worming treatment at all and it improves the immune system so it works at full capacity.
As your vet has given steroids it will take a few months to get them out of the dogs system and for anything else to start to work, the issue with vet meds is that these do stop 'working' ( covering up the problem) and the vet has to continually give stronger meds and all the time the immune system is not recovering so the dog can't fight any issue naturally.