I haven't been to the Mountains in Colorado or anywhere out west. I did got to Peru last year where my wife's family is from originally. My wife had gotten a perscription of something for us to take for the altitude. We started taking when we got to Lima which is down near the cost for a couple days before we went up into the Andes to the towsn where her parents were from. We were as high as about 18,000 in some places. I never got sick while on the perscription, but we hiked up above the town one day and I have never been so out of breath in my life and I'm in pretty good shape. I don't remember what the perscription was but it worked. I don't think I would have a problem at 10,000 or below.