Wheezing Harley has allergies

We took Harely to the vet a few months back for sneezing and wheezing. She was pretty clear that day, it comes and goes. Her doc thinks she has allergies and suggested giving her childrens benadryl. We've never tried it but she's getting worse, happening more often and doesn't seem to be able to clear it out as well..maybe because she is getting older (she is partailly blind and deaf now). Has anyone ever heard of using benadryl? I haven't been able to find anything on the web or a conversion chart of any sort.