Does anyone know if either theophylline or ephedrine, or the two in
combination, can reduce the body's ability to make use of 
available water?  I had kind of an odd experience on a group hike
recently, becoming dehyrated after about 9 hours of rigorous
hiking despite having brought 1 1/2 gallons of water (c. 6 liters).
I drank close to twice as much as anyone else, and no one else was
dehydrated.  I don't think general physical condition was an issue,
since I was in at least the middle of the pack in terms of general
stamina, so far as I could tell.