Alcohol response (flushing & dependence risk)

Your reaction to alcohol is shaped by two enzymes working in sequence. ADH1B turns alcohol into acetaldehyde (a toxic by-product), and ALDH2 clears that acetaldehyde away. A fast ADH1B makes acetaldehyde quickly, while a deficient ALDH2 clears it slowly, and both leave more acetaldehyde around, which causes flushing and tends to lower the risk of alcohol dependence. Because they act on the same chemical from different ends, they are read together.

Two enzymes in sequence

Alcohol is cleared in two steps. ADH1B converts it into acetaldehyde, a toxic and reactive compound, and ALDH2 converts acetaldehyde into harmless acetate. Trouble comes from acetaldehyde building up between those steps, either because the first step is fast or the second is slow.

The ALDH2 variant read here largely disables the second step. One copy leaves roughly a tenth of normal enzyme activity, and two copies almost none, which is why a single copy is enough to produce a clear response. The ADH1B variant speeds up the first step, producing acetaldehyde faster.

What the flush actually is

Facial flushing, warmth, a racing heart, nausea and headache after a small amount of alcohol are acetaldehyde toxicity. It is the same mechanism as disulfiram, the drug given to deter drinking, which works by blocking ALDH2 pharmacologically.

This is one of the most clearly Mendelian common traits in this catalog: with a strong-flush genotype, the response is reliable rather than probabilistic.

The part that matters medically

Acetaldehyde is a recognised carcinogen, and people with reduced ALDH2 function who drink anyway accumulate more of it in the tissues alcohol touches first. Flushers who drink regularly have been reported to carry roughly six to ten times the risk of oesophageal cancer compared with non-flushing drinkers, and the risk rises with intake.

The people at highest risk are not those who avoid alcohol because it makes them ill. They are heterozygotes who build up tolerance and keep drinking, which blunts the warning without removing the acetaldehyde exposure. If drinking makes you flush, that reaction is information rather than an inconvenience to train away.

Why these variants are protective against dependence

Both variants are among the strongest known protective factors against alcohol use disorder anywhere in the genome, precisely because they make drinking unpleasant. That is an unusual kind of protection: it comes from an aversive reaction rather than from any change in the brain's reward system.

Both are also largely East Asian in distribution, which is why alcohol-response genetics reads very differently across populations, and why alcohol flush is often described as an East Asian trait.

One pharmacological footnote

ALDH2 also activates nitroglycerin, the sublingual drug used for angina. People with reduced ALDH2 activity respond less well to it, which is documented and occasionally clinically relevant. It is not something to act on yourself, but it is worth mentioning if nitroglycerin is ever prescribed and seems ineffective.

What each ALDH2 + ADH1B result means

Typical alcohol response. You don't carry the ALDH2 deficiency allele or the fast ADH1B allele, so from what these two genes show you process alcohol at a typical pace without the strong flush-and-nausea reaction. This says nothing about how much you should drink, since tolerance, habit and overall health matter far more, and the usual sensible-drinking guidance still applies.

Fast metabolism, mild aversion (ALDH2 ×0, ADH1B ≥1). Your ALDH2 works normally, but you carry the fast ADH1B variant, so your body converts alcohol into acetaldehyde more quickly and that by-product can pile up briefly. Drinking may feel unpleasant sooner, with some flushing or queasiness, which is linked to a lower chance of heavy drinking or alcohol dependence. The effect is gentler than ALDH2 deficiency because your body still clears the acetaldehyde efficiently.

Moderate flush when drinking (ALDH2 ×1). You carry one copy of the ALDH2 deficiency variant, so the acetaldehyde-clearing enzyme works only partway and alcohol builds up faster than usual, giving the facial flushing, fast heartbeat and queasiness many people feel. The reaction is real but milder than with two copies, and a fast ADH1B (if you also carry it) makes it come on sooner. Drinking less is the simplest way to stay comfortable, and heavier drinking with this genotype still carries some raised cancer risk.

Strong flush, best to avoid alcohol (ALDH2 ×2). You carry two copies of the ALDH2 deficiency variant, so the enzyme that clears acetaldehyde (alcohol's toxic by-product) barely works and even small amounts of alcohol can cause intense flushing, a racing heart and nausea. Beyond the discomfort, regular drinking with this genotype is linked to a markedly higher risk of esophageal cancer, so many people with it avoid alcohol entirely. This is the strongest flush pattern, and your ADH1B variant (if any) only adds to it.

Evidence & sources

This panel reads ALDH2 (rs671) and ADH1B (rs1229984).

Reads the two largest-effect alcohol variants common in East Asia: the strong-flush ALDH2 variant and the fast-metabolism ADH1B variant. Together these are essentially the whole common-variant story for the flush response, though a few rarer alcohol-related variants are not tested.

Common questions

Can I build up tolerance to alcohol flushing? The flush can fade with regular drinking, and that is the concerning part rather than a good sign. Tolerance dulls the symptom without changing the acetaldehyde exposure behind it, and the highest oesophageal cancer risks are reported in people who flush and drink anyway.

Is alcohol flush dangerous by itself? The flush is uncomfortable rather than dangerous. What matters is what it indicates: acetaldehyde, a carcinogen, clearing slowly. The risk comes from continuing to drink, and reported oesophageal cancer risks in flushing drinkers are several times those of non-flushing drinkers.

Why is this so common in East Asia? The ALDH2 variant arose and reached high frequency in East Asia and is rare elsewhere, so both the flush and its protective effect against alcohol dependence are concentrated there. The reasons for its spread are still debated.

Related

Adult lactose digestion · Beta-carotene to vitamin A conversion (BCO1) · Caffeine metabolism speed · Fat taste sensitivity (CD36) · Total cholesterol · Triglycerides · Vitamin D status (low-D genetic burden)