Immunosuppressant Pregnancy: Should not be given to patients who are pregnant or likely to become pregnant in the near future without careful assessment of risk versus benefit; substantial transplacental/transamniotic transmission occurs. Women of childbearing potential should use effective contraception during treatment and for 1 month after; men for 3 months after. Avoid breast-feeding unless benefits outweigh risks.

Azathioprine

Brand names: Imuran, Azathioprine

Azathioprine is an immunosuppressant used in inflammatory bowel disease, autoimmune conditions and transplantation.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Transplants: up to 5 mg/kg/day on day 1; maintenance 1–4 mg/kg/day. Other indications: starting 1–3 mg/kg/day

Route: Oral (IV only when oral route impractical, discontinued as soon as oral tolerated)

Frequency: Once daily

Max: Up to 5 mg/kg/day (transplant, day 1)

Transplants: initial dose up to 5 mg/kg bodyweight/day orally or IV on first day of therapy; maintenance 1–4 mg/kg/day, adjusted to clinical requirements and haematological tolerance; therapy maintained indefinitely because of graft-rejection risk. Other indications: starting 1–3 mg/kg/day, adjusted within these limits to clinical response (may take weeks to months) and haematological tolerance; reduce maintenance to lowest effective level; consider withdrawal if no improvement within 3 months (for IBD consider at least 12 months, response may take 3–4 months); maintenance range <1 to 3 mg/kg/day. Xanthine oxidase inhibitor interaction: when allopurinol given concomitantly, give only 25% of the usual azathioprine dose. TPMT-deficient patients generally require substantial dose reduction; NUDT15 variant patients (esp. homozygotes) generally require dose reduction. Renal or hepatic impairment: consider reducing starting doses; no specific recommendation given. Take with or without food, standardised; at least 1 hour before or 2 hours after milk/dairy products. Paediatric population: posology in children is the same as in adults (transplants and other indications); overweight children may need doses at higher end of range with close monitoring.

Dose adjustments

Renal

Consider reducing starting doses in renal impairment (slower elimination); monitor for dose-related adverse effects. No specific dose recommendation given.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

DOSAGE AND ADMINISTRATION Renal Homotransplantation The dose of azathioprine tablets required to prevent rejection and minimize toxicity will vary with individual patients; this necessitates careful management. The initial dose is usually 3 to 5 mg/kg daily, beginning at the time of transplant. Azathioprine tablets are usually given as a single daily dose on the day of, and in a minority of cases 1 to 3 days before, transplantation. Dose reduction to maintenance levels of 1 to 3 mg/kg daily is usually possible. The dose of azathioprine tablets should not be increased to toxic levels because of threatened rejection. Discontinuation may be necessary for severe hematologic or other toxicity, …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2025-05-19. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Hypersensitivity to azathioprine or to any excipient
Hypersensitivity to 6-mercaptopurine should alert to probable hypersensitivity to azathioprine

Side effects

Bone marrow depression, leukopenia (very common)
Thrombocytopenia (common)
Nausea (common)
Viral, fungal and bacterial infections (very common in transplant patients on combination immunosuppression)
Life-threatening liver injury (rare); hypersensitivity, Stevens-Johnson syndrome and toxic epidermal necrolysis

Interactions

Xanthine oxidase inhibitors (allopurinol) — give only 25% of usual azathioprine dose (allopurinol decreases catabolism)
Ribavirin — co-administration not advised (may reduce efficacy and increase toxicity)
Live organism vaccines — avoid until at least 3 months after end of treatment
TPMT inhibitors (olsalazine, mesalazine, sulfasalazine) — may exacerbate myelosuppression

Clinical monograph

How it works

It is a pro-drug metabolised to 6-mercaptopurine derivatives that impair purine synthesis and lymphocyte proliferation.

Prescribing in practice

Check thiopurine methyltransferase (TPMT) activity before starting — low activity greatly increases the risk of bone-marrow suppression.
Myelosuppression, hepatotoxicity and pancreatitis occur; there is an increased risk of infection and of skin cancer and lymphoma (advise sun protection).
It interacts dangerously with allopurinol and febuxostat (greatly increased toxicity) — the combination needs major dose reduction or avoidance.

Monitoring

Check TPMT before starting; monitor FBC and liver function regularly (frequently at first, then periodically).

Counselling the patient

Attend all your blood tests.
Report a sore throat, fever, bruising, or yellowing of the skin/eyes.
Use sun protection, and tell any clinician you take it before starting a new medicine — especially allopurinol for gout.

Evidence & guidelines

A steroid-sparing immunosuppressant (e.g. NICE inflammatory bowel disease guidance), requiring TPMT testing and blood monitoring.

Reference: BSG IBD Guidelines 2019; NICE NG130 IBD; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways