The Alcohol Flush Reaction, Explained

For many people, even a little alcohol brings flushing and nausea. The ALDH2 gene variant rs671 explains why - and it carries a real health message.

For a lot of people, a single drink brings a hot, red face, a racing heart, and a wave of nausea. This “alcohol flush reaction” is not low tolerance in the casual sense. It is a specific, well-understood genetic difference in how the body processes alcohol.

Two steps to clear alcohol

Your body breaks down alcohol in two main steps. First, alcohol is converted into a compound called acetaldehyde, which is toxic and genuinely unpleasant when it lingers in the bloodstream. Second, an enzyme quickly converts that acetaldehyde into a harmless substance the body can clear. That second enzyme is the key character here: ALDH2.

When step two runs smoothly, acetaldehyde barely gets a chance to build up. When it runs poorly, acetaldehyde accumulates - and your body reacts.

rs671 and the ALDH2*2 variant

The variant behind the flush is rs671, also written Glu504Lys, which produces the allele known as ALDH2*2. It changes one building block of the enzyme, and that small change matters a great deal: the resulting enzyme breaks down acetaldehyde poorly.

With a copy of this variant, drinking alcohol lets acetaldehyde pile up faster than the body can handle. The result is the classic cluster:

  • Facial flushing and redness
  • A feeling of warmth
  • A faster heartbeat
  • Nausea and headache

Because you carry two copies of the gene, the effect tends to be strongest in people with two copies of the variant, and milder but still noticeable with one. If rsIDs are new to you, our guide to what SNPs are covers the groundwork.

Common in East Asian populations

The ALDH2*2 variant is especially common in East Asian populations, which is why the flush reaction is so often described in that context. Its distribution across the world is uneven, a reminder that many traits cluster by ancestry - something you can explore in a general way through origins analysis of your own data.

A genuine health note

This one deserves more than a shrug. The flush reaction is not merely uncomfortable or embarrassing. In people who carry the ALDH2*2 variant and drink heavily, the repeated buildup of acetaldehyde is linked to a higher risk of certain cancers, particularly of the esophagus. Acetaldehyde is the culprit, and this variant lets more of it accumulate over time.

That is not a diagnosis, and it is not a reason to panic - it is a reason to take the reaction seriously rather than pushing through it. If you flush and you have questions about your own drinking and risk, this is exactly the kind of thing to raise with a qualified medical professional.

It is not a tolerance you build

A common misreading is to treat the flush as ordinary low tolerance - something you could train away by simply drinking more. It is closer to the opposite. The reaction reflects a fixed difference in an enzyme, so drinking through it does not teach your body to clear acetaldehyde any better; it just exposes you to more of a toxic compound for longer. It is also not an allergy in the immune sense. It is a metabolic bottleneck, built into how your particular ALDH2 enzyme is shaped.

Finding it in your file

If your raw DNA export includes the marker:

grep '^rs671' my_raw_dna.txt

You will see a two-letter genotype. As always, services can report either DNA strand, so the letters can differ without differing in meaning; a tool that handles orientation clears it up. You can check variants like this with on device DNA analysis that never uploads your file.

This article is educational only and is not medical advice - for anything involving alcohol and your health, please consult a qualified professional.

Further reading