Alcohol dehydrogenase (ADH) is a class of enzymes that catalyze the conversion of alcohols to aldehydes, accompanied by the reduction of NAD+ to NADH. In humans, there are 9 types of ADH isozymes, with most of the ADH activity occurring in the liver. Members of the ADH family are the main enzymes involved in alcohol detoxification; genetic variations in ADH enzymes lead to differences in ADH activity and alcohol tolerance, and may regulate the susceptibility to alcoholism.