1. An unpleasant condition resulting from excessive consumption of alcohol. While their mechanisms are not entirely understood, hangovers are primarily caused by a toxic chemical called acetaldehyde, which is produced when alcohol dehydrogenase enzymes break down ethanol (alcohol) in the liver. Certain other chemicals in distilled spirits, beer, and wine known as congeners may also play a role in causing hangovers. Common symptoms of a hangover include headache, drowsiness, nausea, fatigue, anxiety, dizziness, and general discomfort, among others.