What happens when you take azathioprine with allopurinol?
Azathioprine is a thiopurine immunosuppressant used in inflammatory bowel disease (Crohn's, ulcerative colitis), autoimmune hepatitis, lupus, after organ transplantation, and in several other autoimmune conditions. Allopurinol lowers uric acid and is prescribed for gout and to prevent tumour lysis syndrome. Each is widely used and generally well tolerated on its own. Together, they create a well-documented and potentially fatal interaction.
- Azathioprine becomes 6-mercaptopurine. After absorption, azathioprine is converted to 6-mercaptopurine (6-MP), the molecule that is then routed down several metabolic pathways in the body.
- Two pathways normally dispose of the drug. One enzyme, thiopurine methyltransferase (TPMT), methylates 6-MP into inactive products; another enzyme, xanthine oxidase, oxidises it to 6-thiouric acid for excretion by the kidneys. These routes keep the active drug level in check.
- Allopurinol blocks xanthine oxidase. Allopurinol is a potent xanthine oxidase inhibitor. With that disposal route shut down, more 6-MP is left over.
- Active metabolites build up. The leftover 6-MP is pushed into the activation pathway, raising levels of the active 6-thioguanine nucleotides (6-TGN) that get incorporated into DNA. These are responsible for both the immunosuppressive effect and the toxicity.
- The same azathioprine dose becomes far stronger. The net result is markedly increased immunosuppression and bone-marrow suppression at whatever azathioprine dose the person was taking before allopurinol was added.
This is why the FDA azathioprine label and regulators such as Medsafe New Zealand carry an explicit warning: when allopurinol is used with azathioprine, the azathioprine dose must be substantially reduced and blood counts monitored closely.
Why is this important?
Azathioprine toxicity can develop quickly and be severe. Within days to weeks of adding allopurinol to previously stable azathioprine therapy, a person can develop low white cells (neutropenia), low platelets (thrombocytopenia), and anaemia. When all three blood lines collapse together — pancytopenia — the consequences can be fatal.
Severe neutropenia opens the door to serious infection and sepsis. Severe thrombocytopenia can cause spontaneous bleeding. Severe anaemia can require transfusion. The liver is also at risk: rising thiopurine metabolites can produce raised liver enzymes, cholestasis, and rarely acute liver injury. Mouth ulcers, severe nausea, and pancreatitis are other recognised features of thiopurine toxicity.
The danger is greatest in people with naturally low TPMT activity, whether inherited or drug-related, and in those carrying the NUDT15 variant that is more common in East and South Asian populations. These individuals already face heightened myelosuppression risk at standard azathioprine doses, so adding allopurinol on top can be catastrophic.
One nuance worth understanding: this interaction is sometimes used deliberately. In inflammatory bowel disease, a minority of patients metabolise too much of the drug down the inactive pathway and respond poorly. A specialist may intentionally add low-dose allopurinol alongside a reduced azathioprine dose to redirect metabolism and improve response. This is a closely monitored specialist strategy with enzyme testing — not something to attempt as a casual co-prescription.
What should you do?
The safest approach is to avoid combining these drugs. If you take azathioprine and gout treatment is being considered, ask whether a non-xanthine-oxidase option is suitable for you. If the combination genuinely cannot be avoided, it should be supervised by a specialist with frequent monitoring. Use this schedule as a guide and confirm specifics with your own clinician.
Before any change
- Ask your doctor or pharmacist whether the two drugs can be kept apart — for gout, alternatives that do not block xanthine oxidase may be options.
- Request TPMT and NUDT15 enzyme or genotype testing before starting any thiopurine, especially if combination with allopurinol is on the table.
- Make sure every prescriber and every pharmacy knows you take both medications.
Every day, if you must take both
- Take only the reduced azathioprine dose your specialist has set — do not return to your previous dose.
- Watch for early warning signs: unusual bruising or bleeding, mouth sores, sore throat, fever, marked fatigue, yellowing of the skin or eyes, or right-upper-abdomen pain.
- Do not assume febuxostat is a safe substitute for allopurinol — it blocks the same enzyme and carries the same warning.
After a change is made
- Have complete blood counts checked frequently in the early weeks, then on the ongoing schedule your clinician sets — risk is highest in the first several weeks.
- Have liver function monitored alongside blood counts.
- If any warning sign appears, contact your prescriber urgently rather than waiting for the next scheduled test.
Which specific products are affected?
The interaction applies to azathioprine (sold as Imuran, Azasan, and generic tablets and injection) and to 6-mercaptopurine (Purinethol, Purixan, and generics), which is azathioprine's active metabolite and shares the same xanthine-oxidase clearance route. Both immunosuppressive and cancer uses are affected, though management differs by setting.
The xanthine oxidase inhibitors that cause the problem include allopurinol (Zyloprim, Aloprim, generics) and febuxostat (Uloric, Adenuric); topiroxostat, available in some countries, carries the same concern. This is a class effect — switching from one xanthine oxidase inhibitor to another does not avoid it.
Gout treatments that work by other mechanisms do not produce this interaction. Probenecid acts at the kidney tubule, colchicine works on microtubules, and IL-1 antagonists such as canakinumab act on cytokines. Where appropriate, these may let someone on a thiopurine avoid a xanthine oxidase inhibitor entirely — a question worth raising with your doctor.
The science behind it
The mechanism and its danger are well established across regulatory and clinical sources. The FDA azathioprine prescribing information states that concomitant allopurinol increases mercaptopurine/azathioprine exposure and the risk of myelosuppression, and directs that the azathioprine dose be substantially reduced when the two are used together.
Medsafe New Zealand issued a prescriber bulletin titled "Azathioprine-Allopurinol Interaction: Danger!", explaining that allopurinol's inhibition of xanthine oxidase raises 6-MP levels and can cause potentially fatal blood dyscrasias; it advises against co-prescription unless unavoidable, and if unavoidable, a marked azathioprine dose reduction with frequent blood-count monitoring.
Real-world harm is documented in the clinical literature. A 2020 case report in European Heart Journal — Case Reports described pancytopenia in a heart-transplant patient from the allopurinol–azathioprine interaction (PMID 33442614). A 2021 report in CMAJ described azathioprine-induced severe anaemia potentiated by concurrent allopurinol (CMAJ 2021;193(3):E94). Together these confirm that the xanthine-oxidase block raises active metabolite levels enough to cause serious, sometimes life-threatening bone-marrow suppression in practice, not just in theory.
Frequently Asked Questions
Can I ever take azathioprine and allopurinol at the same time?
Sometimes, but only under specialist supervision with a substantially reduced azathioprine dose and frequent blood and liver monitoring. It is never something to combine on your own or at your usual azathioprine dose.
Is febuxostat a safer alternative to allopurinol here?
No. Febuxostat blocks the same enzyme (xanthine oxidase) and carries the same warning. Switching between xanthine oxidase inhibitors does not avoid the interaction.
What gout treatments are safer if I take azathioprine?
Drugs that work by other mechanisms — such as probenecid, colchicine, or IL-1 antagonists — do not cause this interaction. Ask your doctor whether one of these, or non-drug measures, is appropriate for you.
How soon could a problem appear?
Blood counts can fall within days to weeks of adding allopurinol to stable azathioprine. The risk is highest in the first several weeks, which is why early and frequent monitoring matters.
Why does my doctor want a TPMT or NUDT15 test?
These tests show how well your body inactivates thiopurines. People with low TPMT activity or the NUDT15 variant are at much higher risk of severe myelosuppression, and the result guides safe dosing — especially if allopurinol is involved.
What symptoms mean I should call my doctor urgently?
Unusual bruising or bleeding, mouth sores, fever or sore throat, marked fatigue, yellowing of the skin or eyes, or right-upper-abdomen pain. These can be early signs of bone-marrow suppression or liver injury.
Key takeaways
- Allopurinol blocks the enzyme that disposes of azathioprine, raising its active metabolites and the risk of potentially fatal bone-marrow suppression.
- This is treated as a contraindication: avoid the combination whenever possible.
- Febuxostat is not a safer substitute — it blocks the same enzyme and carries the same warning.
- If the combination is unavoidable, it requires a specialist, a substantially reduced azathioprine dose, TPMT/NUDT15 testing, and frequent blood and liver monitoring.
- Gout treatments that work by other mechanisms (probenecid, colchicine, IL-1 antagonists) may let you avoid the problem entirely.
- Tell every prescriber and pharmacy about both medications — disconnected systems are where this interaction slips through.