ALDH2 rs671: the alcohol flush variant

Genetic marker guide · Updated June 2026

23andMe & raw DNA

Most of what your raw DNA file can tell you about health is probabilistic — a nudge in the odds, not a verdict. ALDH2 is one of the exceptions. The link between the rs671 variant and the alcohol flush reaction is one of the best-understood gene-to-trait connections in all of consumer genetics, with a clear, well-established biological mechanism behind it. That makes it a good place to start if you've never opened your raw data before: the trait is visible in the mirror, and the explanation sits right there in a single line of your file.

The two-step pathway: how your body handles alcohol

Alcohol doesn't leave your body as alcohol. It gets taken apart in two chemical steps, each run by its own family of enzymes.

Step one — ethanol to acetaldehyde. When you drink, enzymes called alcohol dehydrogenases (ADH) convert ethanol into a compound called acetaldehyde. Acetaldehyde is the substance that makes you feel unwell — it's far more toxic than the alcohol you started with, and it's the reason a hangover feels the way it does.

Step two — acetaldehyde to acetate. A second enzyme, ALDH2 (aldehyde dehydrogenase 2), then converts that acetaldehyde into acetate, a harmless compound your body clears easily. ALDH2 is the cleanup crew. As long as it's working, acetaldehyde is a brief intermediate that never gets a chance to build up.

The flush reaction is what happens when step two is broken. If ADH keeps producing acetaldehyde but ALDH2 can't clear it fast enough, acetaldehyde pools in your blood — and your body reacts to it. A useful way to picture it: step one is a tap filling a sink, and step two is the drain. If the drain is narrow, the sink fills up no matter how slowly you pour.

What the flush actually is

The redness isn't an allergy and it isn't sensitive skin. Acetaldehyde causes your blood vessels to dilate, which is why the face, neck, and sometimes the whole upper body turn red. Alongside the flush, people commonly notice a rapid or pounding heartbeat, warmth, headache, nausea, and a stuffy nose. It can come on after a surprisingly small amount of alcohol — sometimes half a drink.

In other words, the flush is a visible readout of acetaldehyde in your bloodstream. That's the key idea to hold onto, because it's also what connects the trait to the health guidance further down: the redder you go, the more acetaldehyde exposure your tissues are getting.

The rs671 variant and why it's common in East Asian ancestry

The gene that codes for ALDH2 has a well-studied variant called rs671 (also written as the Glu504Lys change, or the ALDH2*2 allele). A single amino-acid substitution is enough to largely inactivate the enzyme:

• G allele (the ALDH2*1 version): produces normal, fully active enzyme.
• A allele (the ALDH2*2 version): produces a dysfunctional enzyme — acetaldehyde accumulates.

What makes rs671 unusual is how it behaves in a mix. The A-allele version of the enzyme doesn't just fail on its own; it interferes with the normal G-allele version in the same cell. So carrying even one A allele meaningfully drags down your overall ALDH2 activity — the reason a single copy is enough to cause a real, noticeable reaction.

The A allele is common in people of East Asian descent and comparatively rare in most other populations, which is why the flush is sometimes nicknamed "Asian glow." Population geneticists think the variant spread through East Asian ancestry over the last several thousand years. Ancestry raises or lowers the odds you carry it, but it doesn't decide the matter — the only way to know your genotype is to read it.

What the genotypes are associated with

The pattern lines up with the biology. GG carriers clear acetaldehyde normally. GA carriers — the "one A allele" group — still flush noticeably because the inactive enzyme drags down overall activity. AA carriers have very little functional ALDH2 at all, which is why their reaction is the strongest and their tolerance the lowest. Heterozygous (GA) versus homozygous (AA) is a real difference of degree, not just a footnote: an AA carrier who drinks is dealing with far more acetaldehyde exposure than a GA carrier drinking the same amount.

The acetaldehyde–cancer link, and why it's a reason for moderation

Here is the part worth stating plainly, without alarm. Acetaldehyde is not just unpleasant — it is a recognized carcinogen. Well-established research has consistently found that people who carry the inactive ALDH2 form and continue to drink regularly have an elevated risk of esophageal and other upper-GI cancers, precisely because their tissues are exposed to more acetaldehyde for longer after each drink.

That's the evidence-based case for moderation, and it's why some carriers choose to drink little or not at all. It also reframes the flush from a social nuisance into useful information: the reaction is a built-in signal that your body is accumulating a compound it struggles to clear. Masking that signal — for instance with an antihistamine to reduce the redness — doesn't reduce the underlying acetaldehyde exposure; it only hides the warning light while continuing to drink.

None of this is a diagnosis, and it's not a reason to panic over a past night out. It's a reason to take the trait seriously if you carry it. Heavy drinking also shows up on routine bloodwork — it's a common cause of elevated liver enzymes, where an AST higher than ALT is the classic alcohol-related pattern.

How ADH1B fits in

ALDH2 controls step two, but the first step matters too. A well-studied variant in the ADH1B gene speeds up the first enzyme — the one that turns ethanol into acetaldehyde. A fast ADH1B makes acetaldehyde arrive more quickly; an inactive ALDH2 clears it away more slowly. Someone who happens to carry both a fast-ADH1B variant and an inactive ALDH2 can get an especially intense reaction, because acetaldehyde is produced fast and removed slowly at the same time. The two genes are usually discussed as a pair — for the combined picture, see our companion piece on the ADH1B and ALDH2 alcohol-metabolism genes.

How to find rs671 in your raw data

1. Download your raw data (or from AncestryDNA / MyHeritage).
2. Search it for rs671 and read your genotype.
3. Or use our free DNA explorer — it reads your file in your browser, nothing uploaded.

Strand note: 23andMe typically reports rs671 on the G/A strand (A = the flush-causing allele). If you see an unexpected letter pair, it may be because your file reports the marker on the opposite strand, or because that position came back as a no-call. Raw consumer data is research-grade, not clinical-grade, so a single line is worth reading carefully rather than over-interpreting.

What a flush-positive result should mean to you

If you already flush and your data confirms an A allele, you haven't learned something alarming — you've put a name and a mechanism to something you could already see. What's genuinely useful is the framing it gives you:

• The flush is a real, well-understood signal of acetaldehyde buildup, not a quirk to push through.
• Because acetaldehyde is a carcinogen, moderation is the evidence-based response for carriers who drink — and this is a reasonable thing to raise with a clinician.
• Blunting the visible redness doesn't change the underlying exposure.

What it does and doesn't tell you

ALDH2 is one of the clearer gene-to-trait links in consumer genetics — if you flush, you very likely carry it, and the mechanism behind that is well established. But a few limits still apply. The cancer-risk association is tied to how much you drink, not to the gene alone; a carrier who doesn't drink isn't carrying that particular risk. And 23andMe or AncestryDNA raw data is not a diagnostic test — it reports the letters at a position, which is educational, not a medical result.

The honest takeaway: if you carry the A allele and drink, that's worth raising with a clinician, and the simplest way to act on the biology is to drink less or not at all. Discuss alcohol and your own health with a clinician who can weigh your full picture.

For the full picture of your file, see our complete guide to analyzing 23andMe raw data, or browse the rest of the Quanome blog.

Frequently asked questions

What does ALDH2 rs671 cause?

ALDH2 breaks down acetaldehyde, a toxic byproduct of alcohol. The rs671 A allele produces an inactive enzyme, so acetaldehyde builds up — causing facial flushing, rapid heartbeat, and nausea after drinking (the 'alcohol flush reaction' or 'Asian glow').

Which ALDH2 genotype causes flushing?

GG is the normal, fully active form. GA (one A allele) causes reduced enzyme activity and noticeable flushing. AA causes severe deficiency, strong flushing, and very low alcohol tolerance. The A allele is most common in people of East Asian descent.

How do I find ALDH2 rs671 in my raw data?

Search your raw DNA file for rs671 and read the two-letter genotype, or use a tool that looks it up. 23andMe typically reports this marker on the G/A strand.

Is the alcohol flush reaction dangerous?

The flush itself is uncomfortable but not directly harmful. What matters is why it happens: the flush signals acetaldehyde building up, and acetaldehyde is a recognized carcinogen. Well-established research links regular drinking in people with the inactive ALDH2 form to a higher risk of esophageal and upper-GI cancers. This is educational, not medical advice — discuss your own risk with a clinician.

Why is the ALDH2 A allele so common in East Asian populations?

The rs671 A allele is common in people of East Asian descent and rare in most other populations, which is why the flush reaction is sometimes called 'Asian glow.' It appears to have spread through East Asian ancestry over the last several thousand years. If your ancestry is East Asian, you are more likely to carry it, but ancestry is not destiny — the only way to know your genotype is to look at your data.

Does the flush go away if I keep drinking or take an antihistamine?

Some people find the visible redness lessens with antihistamines or with tolerance, but that only masks the flush — it does not speed up how fast your body clears acetaldehyde. The underlying acetaldehyde exposure is still there. Blunting the warning sign while continuing to drink is the opposite of what the biology suggests.

How does ADH1B interact with ALDH2?

ADH1B controls the first step (ethanol to acetaldehyde) and ALDH2 controls the second (acetaldehyde to acetate). A fast ADH1B variant makes acetaldehyde quickly; an inactive ALDH2 clears it slowly. Someone who carries both can get an especially strong reaction because acetaldehyde is produced fast and removed slowly. The two genes are often discussed together.

Can 23andMe or AncestryDNA diagnose an alcohol-related condition?

No. Consumer raw DNA data is not a diagnostic test. It reports the letters at specific positions like rs671, which is useful for understanding a well-studied trait, but it does not diagnose any disease or tell you your personal cancer risk. Use it as a prompt for an informed conversation with a clinician, not as a medical result.

I flush but I don't see the A allele in my data — what gives?

A few things can cause this: raw data occasionally has a no-call at a given position, files can report a marker on the opposite strand (so the flush allele may appear as a complementary letter), and flushing can have causes unrelated to ALDH2. Raw consumer data is research-grade, not clinical-grade, so treat a single marker cautiously and don't over-read one line.