http://www.wired.com/wiredscience/20...rse-hangovers/
"Brown" liquors contain more nasties than does a decent vodka:
"One reason for the different effects of vodka and bourbon, Rohsenow says, could be that bourbon contains 37 times more toxic compounds than vodka does, including nasty organic molecules such as acetone, acetaldehyde, tannins and furfural. A good rule of thumb for liquors, she says, is that the clearer they are, the less of these substances they contain."
http://www.rupissed.com/hangovercures.html
"Hangover Cures & Remedies
A hangover is caused by a combination of the toxic by-product of alcohol metabolism (acetaldehyde), dehydration, and Vitamin A, B (particularly B6) and C depletion caused by the chemical action of alcohol on your system. The symptoms are usually a dry mouth, nausea, fatigue, dizziness and headache and are caused by a combination of these factors.
Apart from the obvious toxic effects, you should realise you're suffering the effects of a mild overdose of a depressant drug and your nerves are reacting accordingly, and you have also flushed a signifnicant quantity of vitamins and nutrients from your system causing a degree of metabolic shock that your body is struggling to compensate against. This is why hangover symptoms often include disorientation and "the jitters".
Some drinks give you worse hangovers than others due to the actions of impurities called Congeners. While it's the ethyl alcohol (ethanol) that gets you drunk, amyl alcohol, butyl alcohol, methyl alcohol, propyl alcohol, and isopropyl alcohol is also found in most drinks to varying degrees, and the concentration of these Congeners will go a long way to determining the severity of the hangover.
Vodka and gin (clear liquors) are generally lower in congeners than whisky, rum and bourbon, due to the filtering process involved, and hence usually give a less intense hangover. Unfortunately the congeners also give flavor, smell and appearance to alcohol.
Another contributor to the hangover is the "freshness" of the alcohol since the natural oxidation product of ethanol is also acetaldehyde. Finishing off an already open bottle of red, or drinking beer that's been in the boot of your car for a week, will provide you with a ready made supply of acetaldehyde so you don't have to wait for your liver to make it in order to have a really good hangover."