TPMT

Drugs affected by TPMT

Thiopurine S-Methyltransferase

3 medications 4 brand products

About TPMT

TPMT inactivates thiopurine drugs like azathioprine, mercaptopurine, and thioguanine.^[1] About 1 in 300 people of European descent has essentially no TPMT activity, and standard doses can cause life-threatening bone marrow suppression in those people within weeks.^[2] Intermediate activity affects around 10 percent of people and still requires dose reduction.

TPMT testing is one of the oldest and most established pharmacogenetic tests. Most rheumatologists, gastroenterologists, and oncologists order it before starting thiopurine therapy.^[3]

What we test for TPMT

Gene2Rx reports your TPMT genotype across 43 named star alleles, built from 39 variants curated by PharmVar.

Star alleles

Variants tested

PharmVar

Source

GRCh38

Genome build

Normal Function 1 No Function 11 Uncertain Function 27 Unknown Function 4

Notable TPMT alleles

*1 Normal Function

Reference allele — normal TPMT activity.

*3A No Function

The most common no-function allele in European populations; carriers are at high risk for thiopurine toxicity.

~5% in Europeans

*3C No Function

A no-function allele common in African and East Asian populations.

*2 No Function

A rarer no-function allele caused by a single missense variant.

What are star alleles?

Star alleles (like *1, *2, *4) are standardized names for distinct versions of a pharmacogene. *1 is the reference; higher numbers identify variants discovered later that change the enzyme's activity.

You inherit one allele from each parent, so your genotype is a pair (e.g. *1/*4). The pair determines your predicted phenotype — for example, whether you metabolize a drug at a normal, decreased, or no-function rate.

PharmVar is the international registry that defines and curates these allele names. Gene2Rx tests the variants required to call every TPMT allele in the PharmVar catalog.

View the full TPMT allele catalog on PharmVar →

Medications with TPMT guidelines

Gene2Rx covers 3 medications with published pharmacogenetic guidance for TPMT, drawn from CPIC and FDA sources. Each drug links to its full pharmacogenetics page.

chemotherapies →

Mercaptopurine CPIC / FDA Thioguanine CPIC / FDA

immunosuppressants →

Azathioprine CPIC / FDA

Brand products containing a TPMT-affected ingredient

These branded medications include at least one active ingredient whose metabolism or action involves TPMT. Each links to its full pharmacogenetic breakdown.

References

CPIC. CPIC Guideline for Thiopurines and TPMT and NUDT15 (2018). cpicpgx.org
U.S. Food and Drug Administration. Table of Pharmacogenomic Biomarkers in Drug Labeling (2024). fda.gov
PharmGKB / Stanford University. PharmGKB: The Pharmacogenomics Knowledge Base. pharmgkb.org

Find out your personal TPMT phenotype

This page lists drugs affected by TPMT. A Gene2Rx report tells you which metabolizer group you fall into, and what that means for every medication on this list.

Get your report Look up a medication

Informational only, not medical advice. The presence of a TPMT pharmacogenetic guideline does not mean every patient needs to change their dose. Never start, stop, or change a medication without talking to your prescribing clinician.