What Causes a Hangover? What Research Says About Alcohol Metabolism

Hangovers are common, yet research on what causes them is limited. A 2020 review in the Journal of Clinical Medicine by Mackus and colleagues looked at the evidence on alcohol metabolism and oxidative stress.

What a hangover is

The review defines the alcohol hangover as the combination of negative mental and physical symptoms after a single episode of drinking, starting when blood alcohol concentration approaches zero. It notes hangovers can affect memory and thinking, and may make everyday tasks such as driving risky.

How the body breaks down alcohol

According to the review, most alcohol is cleared by the liver in two steps. The enzyme alcohol dehydrogenase turns ethanol into acetaldehyde, and a second enzyme, aldehyde dehydrogenase, turns acetaldehyde into acetate. Oxidative pathways together account for over 90 percent of alcohol elimination, and about 10 percent goes by other routes.

Is acetaldehyde the culprit?

A popular idea blames acetaldehyde. The review says the evidence does not support a direct role. Ethanol crosses the blood-brain barrier freely, but acetaldehyde does not readily cross it, because the barrier is rich in the enzyme that breaks it down. Studies the review describes, including work in 23 healthy men, found no significant link between blood acetaldehyde and hangover severity. The authors add that acetaldehyde is hard to measure because it often falls below detection limits.

What does seem linked

The review says the current evidence suggests a direct relationship between blood ethanol concentration and hangover severity. Fast elimination of ethanol is associated with less severe hangovers. The authors suggest that with slower metabolism, more ethanol reaches the brain.

Oxidative stress, a kind of chemical damage in cells, also showed links, with a mixed pattern. More oxidative stress in the first hours after drinking was linked to a less severe next-day hangover, while more at a later stage was linked to a more severe one.

What is still unknown

The authors conclude that more research is needed on how alcohol metabolism, acetaldehyde and oxidative stress interact. The review also notes that acetate, the end product of breakdown, deserves further investigation.

A note

This article explains the science and is not a guide to drinking. Alcohol harms health, and nothing here is a cure. This is general information, not medical advice.

Sources

• The Role of Alcohol Metabolism in the Pathology of Alcohol Hangover, Mackus and others, Journal of Clinical Medicine, 2020: https://pmc.ncbi.nlm.nih.gov/articles/PMC7692803/

Facts checked: October 15, 2026.