Alcohol dehydrogenase is an enzyme located in the lining of the liver and the stomach that is a catalyst in the oxidation of ethanol into acetaldehyde, producing NADH and a  hydrogen ion as byproducts.  Alcohol dehydrogenase is a dimeric molecule, consisting of two polypeptide chains and containing two active sites. (Protein Data Bank) 

    Alcohol dehydrogenase's most likely evolutionary purpose was to metabolize alcohol in humans produced by the symbiotic bacteria living in the body.  However, now the enzyme is commonly used to metabolize the alcohol in alcoholic beverages.  Ethanol is the only alcohol that can be ingested in moderate amounts without any serious damage to the body.  (Protein Data Bank)

    Ethanol is the alcoholic ingredient in alcoholic beverages, and so alcohol dehydrogenase is capable of decomposing it into relatively harmless compounds.  Relative is used loosely here, as acetaldehyde is more toxic than ethanol.  However, through another reaction with acetaldehyde dehydrogenase, this toxic compound is transformed into the much milder acetic acid.  Acetaldehyde is thought to be the major cause of hangovers, the unpleasant experience involving headaches and nausea occurring after one overindulges.  (Wikipedia)