Systemic Immunosuppressant — Eczema / Autoimmune Dermatoses Pregnancy: Azathioprine 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 and transamniotic transmission occurs and evidence of teratogenicity in man is equivocal. Women of childbearing potential should use effective contraception during treatment and for one month after completion; men are recommended to use effective contraception and not to father a child during treatment and for three months after completion. Reports include intra-uterine growth retardation, premature birth, low birth weight, spontaneous abortion, and leukopenia and/or thrombocytopenia in neonates - extra care in haematological monitoring is advised during pregnancy. Cholestasis of pregnancy has occasionally been reported. Breast-feeding: 6-mercaptopurine has been identified in colostrum and breast milk; women receiving azathioprine should avoid breast-feeding unless the benefit outweighs the potential risks.

Azathioprine

Brand names: Imuran, Azathioprine 25/50mg

Used in: Inflammatory Bowel Disease

Azathioprine is an immunosuppressant used in inflammatory and autoimmune conditions and to prevent transplant rejection.

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: Other (non-transplant) indications: starting dosage 1 to 3 mg/kg bodyweight/day

Route: Oral (take with food or on an empty stomach but standardise the method; at least 1 hour before or 2 hours after milk or dairy products). When the oral route is impractical, azathioprine injection may be given by the IV route only, and this route should be discontinued as soon as oral therapy can be tolerated again.

Frequency: Daily

Max: 3 mg/kg bodyweight/day for other (non-transplant) indications

Dermatological use falls under the SPC 'Other indications' category. The starting dosage should be adjusted within the 1 to 3 mg/kg/day limits depending on the clinical response - which may not be evident for weeks or months - and on haematological tolerance. When therapeutic response is evident, consideration should be given to reducing the maintenance dosage to the lowest level compatible with maintaining that response; the maintenance dosage required may range from less than 1 mg/kg bodyweight/day to 3 mg/kg bodyweight/day. If no improvement occurs within three months, consideration should be given to withdrawing azathioprine. The SPC also gives a separate, higher regimen under a distinct 'Transplants' heading, which does not apply to this page. Specialist medical literature should be consulted for guidance as to clinical experience in particular conditions. XANTHINE OXIDASE INHIBITOR RULE: when allopurinol and azathioprine are given concomitantly it is essential that only 25% of the usual dose of azathioprine is given. TPMT: patients with inherited little or no thiopurine S-methyltransferase activity are at increased risk of severe toxicity from conventional doses and generally require substantial dose reduction; the optimal starting dose for homozygous deficient patients has not been established. Most heterozygous TPMT-deficient patients tolerate recommended doses but some require reduction. NUDT15: patients with an inherited mutated NUDT15 gene are at increased risk of severe toxicity and generally require dose reduction, particularly variant homozygotes; genotypic testing may be considered before initiating therapy and close blood-count monitoring is necessary in any case. DERMATOLOGY-SPECIFIC WARNING (section 4.6): azathioprine and long-wave ultraviolet light have been shown to have a synergistic clastogenic effect. ELDERLY: limited experience; monitor renal and hepatic function and consider dosage reduction if there is impairment. PAEDIATRIC (stated only as a cross-reference, not as a per-kg figure of its own, so not carried in paedDose): the SPC states the posology in children is the same as in adults, for transplants and for other indications alike; children considered to be overweight may require doses at the higher end of the dose range, so close monitoring of response is recommended. Verify any paediatric use against a children's formulary. MONITORING: complete blood counts including platelets weekly or more frequently during the first eight weeks, then at least every 3 months; liver function tests routinely.

Dose adjustments

Renal

Azathioprine pharmacokinetics have not been formally studied in renal impairment, so no specific dose recommendations can be given. Since impaired renal function may result in slower elimination of azathioprine and its metabolites, consideration should be given to reducing the starting dose, and patients should be monitored for dose-related adverse effects. The same applies in hepatic impairment. More frequent blood-count monitoring is advised if severe renal and/or hepatic disorder is present.

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 of the excipients
Hypersensitivity to 6-mercaptopurine should alert the prescriber to probable hypersensitivity to azathioprine

Side effects

Bone marrow depression and leukopenia (very common); thrombocytopenia (common); anaemia (uncommon)
Viral, fungal and bacterial infections (very common in transplant patients on combination immunosuppression; uncommon in other patient populations)
Nausea (common); pancreatitis (uncommon)
Hypersensitivity (uncommon); Stevens-Johnson syndrome and toxic epidermal necrolysis (very rare); alopecia (rare); photosensitivity and acute febrile neutrophilic dermatosis (Sweet's syndrome) (not known)
Cholestasis (uncommon); life-threatening liver injury (rare); neoplasms including lymphoproliferative disorders and skin cancers, melanoma and non-melanoma (rare)

Interactions

Xanthine oxidase inhibitors such as allopurinol - allopurinol decreases the rate of catabolism of azathioprine, so it is essential that only 25% of the usual azathioprine dose is given (section 4.2)
Ribavirin - co-administration is not advised; ribavirin may reduce efficacy and increase toxicity of azathioprine (section 4.4)
Live organism vaccines - potential to cause infection in immunocompromised hosts; patients should not receive live organism vaccines until at least 3 months after the end of treatment (section 4.4)
Medicinal products that inhibit TPMT, such as olsalazine, mesalazine or sulfasalazine - may exacerbate sensitivity to the myelosuppressive effect of azathioprine (section 4.4)
Milk and dairy products - the dose should not be taken with milk or dairy products; take at least 1 hour before or 2 hours after (sections 4.2/4.5)
NOTE: SPC section 4.5 was not captured in full in this bundle; the entries above are those stated within sections 4.2 and 4.4 - the full interaction section must be checked against the SPC

Clinical monograph

How it works

It is metabolised to 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 severe bone-marrow suppression.
It interacts dangerously with allopurinol and febuxostat (xanthine oxidase inhibitors), needing a large dose reduction or avoidance.
There is an increased risk of infection and of certain malignancies, including skin cancer — advise sun protection.

Monitoring

Monitor FBC (and liver function) regularly, especially at initiation and after dose changes; check TPMT before starting.

Counselling the patient

Report sore throat, fever, or unexplained bruising or bleeding.
Tell any prescriber you take it before starting allopurinol or other medicines.
Use sun protection and attend your monitoring blood tests.

Evidence & guidelines

A steroid-sparing immunosuppressant across autoimmune disease and transplantation, with TPMT testing and the critical allopurinol interaction.

Reference: BAD Atopic Eczema Systemic Guidelines 2020; MHRA Drug Safety Update (TPMT); BSR/BAD shared guidelines; 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