I did some searches and didn't find much, so I thought I'd try posting this. The last two times I was in Colorado and did some mountain hikes I really bonked out: tons of gas and in general felt horrible. Hard to really describe, but it seems like when I'm at altitude Imodium has extra effect. It seems to shut down my output for longer than normal, and then after that the gas is much worse than normal.
Even in my barely above sea level home state I notice that I get some gas at the time Imodium is wearing off. I wonder if this is pronounced at altitude?