“Most alcohol is metabolised by the liver, where an enzyme called alcohol dehydrogenase, or AHD, breaks ethanol down into acetaldehyde, which in turn is broken down by another enzyme called acetaldehyde dehydrogenase into acetate, which is then excreted.
“The intermediate product, acetaldehyde, is a toxic chemical that can make a person feel sick. Although under normal conditions acetaldehyde is broken down quite rapidly, if it accumulates in the body, intense feelings of nausea and illness result.”

Why you feel like crap the morning after a big night
Buzzed: The Straight Facts About the Most Used and Abused Drugs from Alcohol to EcstasyCynthia Kuhn Phd, Scott Swartzwelder Phd and Wilkie Wilson Phd
WW Norton and Company (2008)