An oatmeal bath did wonders for our dog when she broke out in hives. We used a different flea medicine, and it caused the allergic reaction. The vet told us to keep an eye on her and use a oatmeal bath to soothe the itching and discomfort. It worked!
Our only problem was keeping the dog from eating her oatmeal bath water. I have also heard of people using a chamomile tea rinse to sooth the itch of hives. I may try it next time.