My boy with respiratory issues responded extraordinarily well to a prescription compounded medicine called Hydroxyzine HCL. Nothing else helped much except Dex which I didn’t want to use long term because of the side effects. Most days you wouldn’t now know he has those issues. He’s now 18 and going strong. It also has helped to have lots of ventilation when he’s stalled (can’t stay out 24/7 because he’s a constant food machine!) and identifying a particular allergen to be aware of. His worst is pine so changing his bedding to pellets as apposed to shavings also helped. If we go to an area where pine trees are pollinating, I have to increase his meds and he will start coughing anyway. Wetting his hay when he’s having issues also seems to help and if using a hay bag keeping it low enough so chaff doesn’t fall in his face. (Did try the steaming route.. “Interesting” when it’s below 0.) I hope that helps even a little!