Why do some people flush when drinking?

Some people flush when drinking alcohol because their body has difficulty breaking down a toxic byproduct of alcohol metabolism called acetaldehyde. When acetaldehyde builds up in the bloodstream, it causes blood vessels in the skin—especially on the face, neck, and chest—to widen, leading to redness, warmth, and sometimes itching or swelling. This reaction is commonly known as alcohol flush reaction.

Alcohol is processed in two main steps. First, alcohol is converted into acetaldehyde, a substance that is more toxic than alcohol itself. Next, acetaldehyde is broken down into acetate, which the body can safely eliminate. In people who flush, this second step works poorly due to reduced activity of a specific metabolic enzyme. As a result, acetaldehyde accumulates quickly after drinking, triggering flushing and other symptoms such as nausea, headache, rapid heartbeat, or dizziness.

This reaction is most often genetic and is especially common in people of East Asian descent, though it can occur in anyone. It is not an allergy, but a metabolic response that reflects how the body handles alcohol. Importantly, flushing is not harmless. Elevated acetaldehyde levels place additional stress on the body and are associated with a higher risk of certain alcohol-related health problems, even at lower levels of drinking.

Many people assume flushing is just a cosmetic or uncomfortable side effect. In reality, it is a visible signal that alcohol is being processed inefficiently, which can help explain why drinking feels different—or worse—for some individuals than for others.