"The presence of alcohol in the blood interferes with the normal use of oxygen by the tissues (histotoxic hypoxia). Because of reduced pressure at high altitudes and the reduced ability of the hemoglobin to absorb oxygen, the effect of alcohol in the blood, during flight at high altitudes, is much more pronounced than at sea level. The effects of one drink are magnified 2 to 3 times over the effects the same drink would have at sea level."
http://www.allstar.fiu.edu/aerojava/HumFac04.htm