Just for clarification, altitude sickness and dehydration are two different issues. Being at altitude requires you to increase water intake due to increased "dehydration factors", but that is not altitude sickness. Dehydration causes headaches and altitude sickness causes headaches, but other than that they are different.
Yes, you need to consume a lot more water, but that will not prevent altitude sickness. Everyone's body reacts differently to altitude. The only real treatment for altitude is the body spending time in it to acclimate. There is a prescription that is supposed to help with symptoms, but I haven't tried it. I plan to next time I go. The other treatment is to drop down in altitude. Even going down a 1000' can make a big difference.
I get headaches everytime I go to altitude. I've been out for 10 days and still had headaches the last day. I also don't sleep well. I really hate when people tell me to drink more water to fix it. I've drank so much I had to pee every 20 minutes.
One year while skiing at Breckenridge I had it really bad and probably should have gone to the ER. It probably didn't help that I went all the way to the top to ski the first day 13K plus feet. I felt sick, tossed and turned all night, my heart rate was 110-125 bpm resting and my O2 levels were in the low 80s. I went to outpatient the next morning, but I was doing better by then.
One other thing I've found to help is to take aspirin. The baby pilla day ahead of time, and then take aspirin instead of ibuprofen when you do get headaches. I think the thinning of the blood helps somehow, but it does better for the headaches.
Sorry for the long rant, but I'm familiar and sensitive to this topic and there's a lot of misunderstanding on the subject.
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