Is Purinethol affected by genetics?

Yes — the active ingredient is metabolized by a gene known to vary between individuals.

Relevant genes: NUDT15, TPMT

Purinethol is affected by pharmacogenetics through the NUDT15 and TPMT genes. Your genotype for these genes can change how your body processes Purinethol, which can affect both how well it works and how well you tolerate it. The strongest evidence level on this page is Strong, based on CPIC or FDA guidelines.

What's in Purinethol

mercaptopurine affected by NUDT15, TPMT

Affected by NUDT15, TPMT · CPIC, FDA · Strong evidence

Read the full mercaptopurine genetics guide →

Mercaptopurine phenotype recommendations

Published guidance from CPIC and FDA on how mercaptopurine should be dosed or substituted based on your TPMT, NUDT15 phenotype.

Phenotype	What it means	Recommendation	Evidence
Normal Metabolizer TPMT	Your genotype supports standard dosing.	CPIC Initiate therapy with standard starting dose of mecaptopurine (e.g., 75 mg/m2/day for malignancy or 1.5 mg/kg/day for nonmalignancy). During therapy, adjust the doses of myelosuppressive agents, as per standard clinical practice. It usually takes at least 2 weeks of stable dosing to reach steady state after each dose adjustment.	Strong
Intermediate Metabolizer TPMT	Your genotype reduces your ability to process mercaptopurine; a lower starting dose is typically recommended.	CPIC Initiate therapy with decreased starting doses (30-80% of standard starting dose) if starting dose is ≥75 mg/m2/day (for malignancy) or ≥1.5 mg/kg/day (for nonmalignancy). If starting dose is already below standard starting dose, dose reduction might not be necessary. During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 2-4 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, and the patient is on combination therapy, emphasis should be on reducing mercaptopurine over other agents.	Strong
Possible Intermediate Metabolizer TPMT	Your genotype reduces your ability to process mercaptopurine; a lower starting dose is typically recommended.	CPIC Initiate therapy with decreased starting doses (30-80% of standard starting dose) if starting dose is ≥75 mg/m2/day (for malignancy) or ≥1.5 mg/kg/day (for nonmalignancy). If starting dose is already below standard starting dose, dose reduction might not be necessary. During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 2-4 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, and the patient is on combination therapy, emphasis should be on reducing mercaptopurine over other agents. FDA Reduce dose based on tolerability per FDA labeling. Monitor closely for myelosuppression.	Strong
Poor Metabolizer TPMT	Your genotype greatly reduces your ability to process mercaptopurine and substantially increases the risk of severe bone marrow toxicity. An alternative drug is typically recommended.	CPIC For malignancy: initiate therapy with drastically reduced starting doses. Reduce starting dose by 10-fold and reduce frequency to thrice weekly instead of daily (e.g. 10 mg/m2/day given 3 days/week). During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 4-6 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, emphasis should be on reducing mercaptopurine over other agents. For nonmalignancy: consider alternative nonthiopurine immunosuppressant therapy.	Strong
Indeterminate TPMT	We could not confidently determine your TPMT genotype; standard dosing and close monitoring for toxicity is recommended.	CPIC Based on NUDT15, initiate therapy with standard starting dose of mecaptopurine (e.g., 75 mg/m2/day for malignancy or 1.5 mg/kg/day for nonmalignancy). During therapy, adjust the doses of myelosuppressive agents, as per standard clinical practice. It usually takes at least 2 weeks of stable dosing to reach steady state after each dose adjustment. FDA Initiate therapy with recommended starting dose.	Strong
Not available TPMT	We do not have a TPMT result for you, so we cannot personalize guidance for mercaptopurine.	CPIC Use standard mercaptopurine dosing and monitor closely for bone marrow toxicity. FDA Initiate therapy with recommended starting dose.	—
Normal Metabolizer NUDT15	Your genotype supports standard dosing.	CPIC Initiate therapy with standard starting dose of mecaptopurine (e.g., 75 mg/m2/day for malignancy or 1.5 mg/kg/day for nonmalignancy). During therapy, adjust the doses of myelosuppressive agents, as per standard clinical practice. It usually takes at least 2 weeks of stable dosing to reach steady state after each dose adjustment. FDA Initiate therapy with recommended starting dose.	Strong
Intermediate Metabolizer NUDT15	Your genotype reduces your ability to process mercaptopurine; a lower starting dose is typically recommended.	CPIC Initiate therapy with decreased starting doses (30-80% of standard starting dose) if starting dose is ≥75 mg/m2/day (for malignancy) or ≥1.5 mg/kg/day (for nonmalignancy). If starting dose is already below standard starting dose, dose reduction might not be necessary. During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 2-4 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, and the patient is on combination therapy, emphasis should be on reducing mercaptopurine over other agents. FDA Reduce dose based on tolerability per FDA labeling. Monitor closely for myelosuppression. More substantial reductions may be needed if intermediate for both TPMT and NUDT15.	Strong
Possible Intermediate Metabolizer NUDT15	Your genotype reduces your ability to process mercaptopurine; a lower starting dose is typically recommended.	CPIC Initiate therapy with decreased starting doses (30-80% of standard starting dose) if starting dose is ≥75 mg/m2/day (for malignancy) or ≥1.5 mg/kg/day (for nonmalignancy). If starting dose is already below standard starting dose, dose reduction might not be necessary. During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 2-4 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, and the patient is on combination therapy, emphasis should be on reducing mercaptopurine over other agents. FDA Reduce dose based on tolerability per FDA labeling. Monitor closely for myelosuppression.	Strong
Poor Metabolizer NUDT15	Your genotype greatly reduces your ability to process mercaptopurine and substantially increases the risk of severe bone marrow toxicity. An alternative drug is typically recommended.	CPIC For malignancy: initiate therapy with drastically reduced starting doses. Reduce starting dose by 10-fold and reduce frequency to thrice weekly instead of daily (e.g. 10 mg/m2/day given 3 days/week). During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 4-6 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, emphasis should be on reducing mercaptopurine over other agents. For nonmalignancy: consider alternative nonthiopurine immunosuppressant therapy. FDA Drastically reduce initial dose to 10% or less of recommended dosage per FDA labeling. Consider alternative therapy. Monitor closely for myelosuppression.	Strong
Indeterminate NUDT15	We could not confidently determine your NUDT15 genotype; standard dosing and close monitoring for toxicity is recommended.	CPIC Based on TPMT, initiate therapy with standard starting dose of mecaptopurine (e.g., 75 mg/m2/day for malignancy or 1.5 mg/kg/day for nonmalignancy). During therapy, adjust the doses of myelosuppressive agents, as per standard clinical practice. It usually takes at least 2 weeks of stable dosing to reach steady state after each dose adjustment. FDA Initiate therapy with recommended starting dose.	Strong
Not available NUDT15	We do not have a NUDT15 result for you, so we cannot personalize guidance for mercaptopurine.	CPIC Use standard mercaptopurine dosing and monitor closely for bone marrow toxicity. FDA Initiate therapy with recommended starting dose.	—
Normal Function TPMT	Your body processes mercaptopurine at a normal rate based on this gene, so the standard dose should be appropriate for you.	FDA Initiate therapy with recommended starting dose.	Strong
Intermediate Function TPMT	Your body breaks down mercaptopurine more slowly than normal, which increases the risk of a weakened immune system and low blood cell counts. A lower dose is recommended with close blood count monitoring.	FDA Reduce dose based on tolerability per FDA labeling. Monitor closely for myelosuppression. More substantial reductions may be needed if intermediate for both TPMT and NUDT15.	Strong
Poor Function TPMT	Your body has great difficulty breaking down mercaptopurine, putting you at serious risk of dangerously low blood cell counts. You will need a drastically reduced dose or a different medication entirely.	FDA Drastically reduce initial dose to 10% or less of recommended dosage per FDA labeling. Consider alternative therapy. Monitor closely for myelosuppression.	Strong

Sources: CPIC, FDA

The genes behind the guidance

NUDT15 Nudix Hydrolase 15

NUDT15 protects cells from thiopurine chemotherapy by breaking down active thioguanine nucleotides before they cause too much damage. Loss-of-function NUDT15 variants are common in East Asian and Hispanic populations and cause severe bone marrow suppression on standard thiopurine doses. NUDT15 testing is now considered as essential as TPMT testing before starting azathioprine, mercaptopurine, or thioguanine.

The 2025 CPIC update emphasizes interpreting TPMT and NUDT15 together: patients who are intermediate on both genes need a more aggressive dose reduction than either gene alone would suggest.

See all drugs affected by NUDT15 →

TPMT Thiopurine S-Methyltransferase

TPMT inactivates thiopurine drugs like azathioprine, mercaptopurine, and thioguanine. 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. 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.

See all drugs affected by TPMT →

Browse the full drug-class: Chemotherapy agents.

Related medications

Find out how your genetics affect Purinethol

This page describes the general pharmacogenetics. A Gene2Rx report analyzes your own DNA to tell you which metabolizer group you fall into, across every medication.

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Informational only — not medical advice. Pharmacogenetic guidance describes population-level patterns; your individual response depends on many factors. Never start, stop, or change a medication without talking to your prescribing clinician.