TriHist and/or other allergy treatments

I'm wondering if anyone has had experience with TriHist? How well it worked? If you felt another type of treatment was more effective?

I'm considering putting Bailee on a preventative medication from the inside out this year. She has such a severe reaction to the bugs and I hate seeing her suffer for 6 months out of the year. (itchiness and hives that result in baldness from chest back to teets. Bald tail from rubbing. Bald spots in mane and forelock from rubbing).

We've tried being super aggressive with ointments and sprays once it starts.. (to prevent flies/knats/etc)....... but this year I'd like to attack it before it starts.

Do you know what the triggers are? I tried Tri-Hist one summer and it didn't work. It didn't work because at the time I was treating her for sweet itch because her symptoms were classic. I hadn't figured out that grass was actually her trigger so she was still out in the pasture. I didn't matter what I tried on her because she was standing 24/7 in it. What works for me is first drylotting in the summer, lots of ground flax (1/2# a day) a box fan in her stall and when it gets unbearable, 2-3 mls of Dex every 2-3 days.