The reason for a red face when drinking is the lack of acetaldehyde dehydrogenase in the human body. The metabolic process of alcohol in the human body is from ethanol dehydrogenase to acetaldehyde, acetaldehyde is changed to acetic acid by acetaldehyde dehydrogenase, and finally metabolized to water and carbon dioxide. When acetaldehyde dehydrogenase is reduced, the product of ethanol metabolism, acetaldehyde, will accumulate in the body, resulting in an increase in acetaldehyde. Acetaldehyde has a vasodilating effect, so it can cause flushing of the face. The amount of acetaldehyde dehydrogenase in each person’s body is different, or the activity of the enzyme is different, so the reaction is different. Generally speaking, the redness of the face when drinking alcohol is caused by the lack of this enzyme.