Our miracle cure has been Claretin - actually, the generic Lortadine 10mg. The dogs gets a half tablet each day and it really does the trick. Best of all - no noticable side effects and it's safe for long-term use - unlike steroids. Bella is 25 pounds and Akachan weighs 12, and the dose works fine for both of them.
Obviously, if you have questions or concerns, you should talk to your vet. But if you are already giving your dog antihistimines, you might want to try the Lortadine. Just make sure you don't get the kind with the decongestant added. It's easy and inexpensive, and if your dog is suffering, it's worth a try.