Alcohol dehydrogenases (ADH) are a family of enzymes that catalyzes the conversion of alcohols to aldehydes with the concomitant reduction of NAD to NADH. In humans there are nine isozymes of ADH with the majority of ADH activity occurring in the liver. ADH family members are the primary enzymes involved in alcohol detoxification. Genetic variations in ADH enzymes result in differences in ADH activity and tolerances for alcohol and may regulate susceptibility to alcoholism.