My guy has gotten around 20,000 mg a day, 10,000 mg in the AM and 10,000 mg in the PM in the past when he got bad. I did it the whole summer until the snow hit the ground.
I was able to find Anihist at the farm supply, guess I better head back and stock up on it! When he's doing alright, he gets 10,000 mg MSM (in conjunction with the Anihist usually).
All the other cough syrups and supplements short of dexamethasone/hydrocortisone shots and clenbuteral didn't do a thing for him. I think the Smartbreathe may have helped a bit, but I added extra MSM to it too.
I use 15,000 mg daily of MSM to prevent/limit winter and spring horse sneezing and it works great. Also helps with stocking up when horse is stalled due to bad weather. Have a friend that uses smartpak's SmartBreathe supplement on COPD horse with fabulous results.
My guy develops a cough in spring too. I put him on MSM and cough free. He won't touch anything in his feed. I mix it in apple sauce and make a paste, then syringe it. 1x a day until the weather stays consistant. Works like a charm!
I have had very good results with using SmartPak's Smart Breathe. Available as powder or pellets, powder is a little cheaper and very palatable so I use it. It has MSM , Quercetin and Bromelain, plus other ingredients. The Querceting/Bromelain combo is a great anti-inflamnatory and used by lot's of humans for allergy relief. I find just that combo more effective than MSM. But the Smartbreathe mix is quite good and is just over $1 per day for a single dose.