I haven't tried Tri-Hist, yet. There is a fairly new product called Re-Covr that I may ask my vet about. The generic name is, and I've seen it spelled 2 different ways, is Tripelennamine or Tripplennamine. Here's a couple of links:
Hmmmm, this one sppears to be an injection, which would be good if they're stung by something and have a reaction, but I don't think I'd wanna have to give a shot all the time. I also couldn't find any information about the dosage, like how often you'd have to give it and how much. Let me know what you find out from your vet.