If your dog is given antihistamines for their condition I really recommend that you let them stay for a few days at the vet's kennel. Our dog had a pretty bad reaction to some vaccinations and had even worse luck when it came to the antihistamines he was given.
Basically our dog had a seizure after being given an antihistamine. As we had left our dog with the vet he made a full recovery. But it was a really scary experience. I can't imagine what would have happened if our dog had been at home. I know what to do for a person when they have a seizure, but I can't imagine how you would help your pet.