Drug acetylation speed (NAT2)

NAT2 is a liver enzyme that breaks down several drugs (like the TB antibiotic isoniazid, some sulfa drugs and hydralazine) by 'acetylating' them. People are 'fast' or 'slow' acetylators, which changes drug levels and side-effect risk.

What NAT2 acetylation speed means

NAT2 attaches an acetyl group to certain drugs and chemicals, which is one of the body's main routes for deactivating and excreting them. People fall into slow, intermediate and fast acetylator groups, and the split is old enough that it was recognised from drug responses in the 1950s, long before the gene was found.

This variant is a tag for the wider NAT2 haplotype rather than a functional change itself, which is a limitation worth carrying into any interpretation of the result.

Where acetylator status has real consequences

Isoniazid, a first-line tuberculosis drug, is the classic case: slow acetylators reach higher drug levels and have more liver toxicity and peripheral neuropathy, while fast acetylators can be underdosed at standard regimens. Hydralazine and procainamide, both of which can trigger drug-induced lupus, and sulfamethoxazole are also NAT2 substrates.

Caffeine is metabolised partly through the same route, which is why NAT2 activity is sometimes measured using caffeine as a probe.

The bladder cancer association

Slow acetylation is associated with a modestly higher risk of bladder cancer, because NAT2 deactivates aromatic amines, the carcinogens found in tobacco smoke and some industrial dyes. In a study of 356,027 people, the catalogued association here was an odds ratio of about 1.16.

The important part is the interaction rather than the number: the association concentrates in people exposed to those chemicals, primarily smokers and workers in certain industries. For a non-smoker with no occupational exposure it has little to act on, and for a smoker the actionable factor is the smoking.

What this variant does not tell you

Acetylator status is properly determined by the combination of several NAT2 positions, and a single tag variant classifies people imperfectly, particularly outside European-ancestry populations where haplotype patterns differ. Clinical laboratories that assess NAT2 read multiple positions.

No major guideline currently recommends NAT2 genotyping before prescribing isoniazid, although the association is well documented and genotype-guided dosing has been trialled. This is a result that explains a pattern rather than one that changes a prescription.

What each rs1495741 genotype means

rs1495741 has three possible genotypes: GG, AG and AA.

Faster acetylator (NAT2) (rs1495741 GG). By this NAT2 tag you lean toward being a 'fast acetylator', clearing drugs like isoniazid and some sulfa medicines relatively quickly. This mostly matters around those specific drugs, and a tag SNP gives the general tendency rather than a precise phenotype.

Intermediate acetylator (rs1495741 AG). By this NAT2 tag you fall in the intermediate range for acetylating drugs like isoniazid. Acetylator status mainly matters around those specific medicines, and this single marker is an approximation of the full NAT2 picture.

Slower acetylator (NAT2) (rs1495741 AA). By this NAT2 tag you lean toward being a 'slow acetylator', so drugs cleared by NAT2 (such as the TB antibiotic isoniazid, hydralazine, or some sulfonamides) can build up to higher levels and cause more side effects. It's worth knowing if you're ever prescribed those, though a tag SNP is only an approximation of the full picture.

Evidence & sources

Across global populations, about 51% of people carry two copies of this variant, and about 9% carry none.

This variant appears in 3 published genetic studies: PubMed 20972438 · PubMed 24163127 · PubMed 37210288.

Common questions

Am I a slow or fast acetylator? This variant tags the acetylator haplotype and classifies most people of European ancestry reasonably well, but it is a proxy rather than a full readout, and it performs less well in other populations. Laboratories that assess NAT2 clinically read several positions.

Does slow acetylation mean I should avoid any drugs? No drug is contraindicated on this basis. It is associated with more side effects from isoniazid at standard doses, which is a monitoring question for a clinician treating tuberculosis, not something to act on in advance.

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