Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
AZATHIOPRINE vs ENVARSUS XR
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Azathioprine is a purine analog that inhibits purine nucleotide synthesis, thereby suppressing DNA replication and cell proliferation. It is converted to 6-mercaptopurine, which acts as a purine antagonist, inhibiting de novo purine synthesis and interfering with RNA and DNA synthesis, particularly in rapidly dividing cells such as T-lymphocytes.
Calcineurin inhibitor. Binds to FKBP-12, forming a complex that inhibits calcineurin phosphatase, thereby blocking T-cell activation and IL-2 transcription.
Renal transplant rejection prophylaxis (FDA),Rheumatoid arthritis (FDA),Off-label: autoimmune hepatitis, systemic lupus erythematosus, inflammatory bowel disease (Crohn's disease, ulcerative colitis), pemphigus, myasthenia gravis, dermatomyositis/polymyositis,Off-label: myelodysplastic syndrome, refractory immune thrombocytopenic purpura, atopic dermatitis, Behçet's syndrome
Prophylaxis of organ rejection in kidney transplant patients,Prophylaxis of organ rejection in liver transplant patients
1.5 to 2.5 mg/kg orally once daily; typical adult dose 50-150 mg/day orally. Intravenous dose is 3-5 mg/kg/day as a slow infusion over 30-60 minutes.
0.2 mg/kg/day orally once daily, with the morning meal, using extended-release tablets. Dose adjustments guided by trough concentrations.
Terminal elimination half-life of azathioprine is approximately 2–5 hours; its active metabolite 6-mercaptopurine has a half-life of 1–2 hours, but 6-thioguanine nucleotides accumulate in red blood cells with a half-life of several days, correlating with myelosuppression.
Terminal half-life approximately 25-30 hours in stable renal transplant patients. Longer half-life (up to 50 hours) in patients with hepatic impairment.
Azathioprine is metabolized by xanthine oxidase (XO) and thiopurine methyltransferase (TPMT) to active (6-mercaptopurine) and inactive metabolites. 6-Mercaptopurine is further metabolized by XO to 6-thiouric acid and by TPMT to 6-methylmercaptopurine. Genetic deficiency of TPMT increases risk of toxicity.
Primarily hepatic via CYP3A4 and CYP3A5; also metabolized by intestinal CYP3A4.
Renal (approximately 2% as unchanged drug, 30% as 6-thiouric acid and other metabolites); biliary/fecal (minor, <10% as metabolites).
Primarily fecal (94%) with minor renal excretion (2.2% as unchanged drug). Biliary excretion is a significant route.
Approximately 30% bound, primarily to albumin.
Approximately 99% bound to erythrocytes and plasma proteins, primarily albumin and alpha-1-acid glycoprotein.
0.8–1.0 L/kg, indicating distribution into total body water; extensive distribution into tissues including liver and erythrocytes.
0.9-1.4 L/kg in renal transplant patients; large volume indicates extensive tissue distribution, particularly to red blood cells.
Oral bioavailability of azathioprine is 60–80% (mean 70%) with interindividual variability; absorption may be reduced by food.
Oral bioavailability is approximately 15-25% with the extended-release formulation; reduced by high-fat meal, so should be taken consistently on an empty stomach.
GFR >50 m L/min: no adjustment. GFR 10-50 m L/min: administer 75% of normal dose. GFR <10 m L/min: administer 50% of normal dose. Hemodialysis: administer 50% of normal dose after dialysis.
No specific GFR-based dose adjustment; however, due to nephrotoxicity, monitor renal function closely and reduce dose if renal impairment occurs. For patients with severe renal impairment (Cr Cl <30 m L/min), consider alternative immunosuppression.
Child-Pugh Class A: no adjustment. Child-Pugh Class B: reduce dose by 25-50%. Child-Pugh Class C: contraindicated or use with extreme caution; reduce dose by at least 50%.
In patients with mild to moderate hepatic impairment (Child-Pugh A or B), reduce dose by 25%. For severe hepatic impairment (Child-Pugh C), reduce dose by 50% and monitor trough levels closely.
1.5 to 2.5 mg/kg orally once daily; maximum 150 mg/day. For inflammatory bowel disease: 2-3 mg/kg/day. Intravenous: 3-5 mg/kg/day as a slow infusion.
For pediatric kidney transplant recipients: 0.2 mg/kg/day orally once daily, with morning meal. Adjust to target trough concentrations. Safety and efficacy not established for other indications in pediatrics.
Initiate at lower end of dosing range (1.5 mg/kg/day) due to potential for decreased renal and hepatic function; monitor renal function and hematologic parameters closely.
No specific dose adjustment; however, elderly patients may have increased susceptibility to nephrotoxicity and neurotoxicity. Use lowest effective dose, monitor renal function, and adjust based on trough levels.
Malignancy: Patients receiving immunosuppressive therapy including azathioprine have an increased risk of developing lymphoma and other malignancies, particularly skin cancers. The risk is related to the duration and intensity of immunosuppression. Hematologic toxicity: Severe leukopenia, thrombocytopenia, and anemia, which may be dose-related, can occur. Regular monitoring of blood counts is required. Hepatotoxicity: Hepatotoxicity, including fatal liver injury, has been reported, particularly at high doses.
Increased susceptibility to infection and possible development of malignancy (e.g., lymphoma, skin cancer).
Hematologic monitoring: regular CBCs; Increased risk of infection; Hepatotoxicity; Pancreatitis; Carcinogenicity (lymphoma, skin cancer); TPMT deficiency increases myelotoxicity; Vaccination (live vaccines contraindicated); Renal and hepatic impairment; Drug interactions: allopurinol (reduce dose by 75%), ACE inhibitors (anemia), warfarin (anticoagulant effect decreased).
Nephrotoxicity, neurotoxicity, hypertension, hyperkalemia, post-transplant diabetes mellitus, monitoring of blood concentrations required.
Hypersensitivity to azathioprine or 6-mercaptopurine; Pregnancy (unless benefit outweighs risk) - Category D; Lactation; Patients with TPMT deficiency (increased risk of severe myelotoxicity); Severely depressed bone marrow function; Active infections; Concurrent use of live vaccines; Pre-existing malignancy (except in organ transplantation context).
Hypersensitivity to tacrolimus or any component of the formulation.
No known significant food interactions. Avoid grapefruit juice? (No interaction reported). Maintain consistent diet; no specific restrictions. Limit alcohol due to hepatotoxicity risk.
Grapefruit and grapefruit juice increase tacrolimus exposure and must be avoided. High-fat meals may decrease absorption; consistency of food intake relative to dosing is recommended. Alcohol should be limited due to potential additive hepatotoxicity.
Azathioprine is pregnancy category D. First trimester: increased risk of congenital anomalies including ventricular septal defects, limb defects, and esophageal atresia. Second and third trimesters: risk of intrauterine growth restriction, preterm birth, and neonatal immunosuppression (leukopenia, thrombocytopenia). Use only if benefit outweighs risk.
Envarsus XR (tacrolimus) is classified as FDA Pregnancy Category C. In the first trimester, there is an increased risk of congenital anomalies (e.g., cardiac, renal) based on animal studies; human data are limited but suggest a possible small increase. During the second and third trimesters, risks include intrauterine growth restriction, preterm delivery, and transient neonatal hyperkalemia and renal dysfunction. Advise women of childbearing potential to use effective contraception.
Azathioprine is excreted into breast milk in low concentrations. M/P ratio is approximately 0.7. Nursing infants of mothers on azathioprine have not shown adverse effects; however, theoretical risk of immunosuppression exists. Caution is advised; monitor infant for increased infections.
Tacrolimus is excreted into human breast milk. The milk-to-plasma ratio is approximately 0.5 (range 0.12–0.75). Infant exposure is estimated to be <1% of the maternal weight-adjusted dose, which is considered low. However, due to potential for immunosuppression and adverse effects, breastfeeding is generally not recommended unless benefits outweigh risks. Monitor infant for signs of immunosuppression.
Pharmacokinetic changes in pregnancy include increased clearance and decreased absorption. Dose may need adjustment to maintain therapeutic efficacy. Close monitoring of disease activity and drug levels (6-thioguanine nucleotide levels) is recommended. No standard dose adjustment; individualization required.
Pregnancy induces pharmacokinetic changes including increased volume of distribution, altered protein binding, and enhanced clearance of tacrolimus. Frequent monitoring of trough concentrations is essential to maintain therapeutic levels (target 5–10 ng/m L). Dose adjustments (increases of 20–50% or more) are often required, especially during the second and third trimesters. Postpartum, doses should be reduced to pre-pregnancy levels within 1–2 weeks.
Monitor CBC and LFTs weekly for first month, then biweekly for 2 months, then monthly. TPMT genotype testing before initiation. Avoid concurrent allopurinol unless dose reduced to 25% of original. Use with caution in renal impairment. May cause hepatotoxicity, pancreatitis, or lymphoproliferative disorders.
ENVARSUS XR is an extended-release formulation of tacrolimus; conversion from immediate-release tacrolimus requires close therapeutic drug monitoring due to altered pharmacokinetics. Administer consistently with or without food to minimize variability. Avoid grapefruit products. Monitor renal function, blood pressure, electrolytes, glucose, and trough tacrolimus levels. CYP3A4/5 inducers/inhibitors significantly affect tacrolimus exposure; adjust dose accordingly. Do not crush, chew, or split tablets.
Take exactly as prescribed; do not double dose if missed.,Avoid live vaccines during treatment and for 3 months after stopping.,Report any signs of infection, unexplained bruising/bleeding, or jaundice immediately.,Limit sun exposure and use sunscreen due to increased skin cancer risk.,Do not take allopurinol or other new medications without consulting doctor.,Maintain adequate hydration to reduce risk of hepatotoxicity.,Regular blood tests are required to monitor for side effects.
Take exactly as prescribed, at the same time each day, with or without food but consistently.,Swallow whole; do not crush, chew, or break the tablet.,Avoid grapefruit and grapefruit juice.,Do not stop or change dose without consulting your doctor.,Report signs of infection (fever, sore throat), tremor, headache, changes in urination, or unusual bleeding.,Avoid live vaccines and limit sun exposure due to increased skin cancer risk.,Keep all appointments for blood tests to monitor drug levels and organ function.
"Azathioprine may reduce the therapeutic efficacy and cardiotoxic effects of digitoxin by accelerating its metabolism through induction of cytochrome P450 enzymes, particularly CYP3A4. This interaction can lead to decreased digitoxin serum concentrations, potentially resulting in loss of heart rate control in patients with atrial fibrillation or heart failure. Conversely, the cardiotoxic risk of digitoxin is diminished, but the therapeutic goal may be compromised."
"Azathioprine and fingolimod both suppress lymphocyte function, leading to additive or synergistic immunosuppression. This combination increases the risk of severe infections, including opportunistic infections, due to profound immune system suppression. Clinically, patients may present with prolonged lymphopenia, increased susceptibility to infections, and potential reactivation of latent viruses such as JC virus (causing progressive multifocal leukoencephalopathy) or cytomegalovirus."
"Azathioprine, an immunosuppressant that acts as a prodrug for 6-mercaptopurine, can increase the myelosuppressive effects of benazepril, an ACE inhibitor. This interaction is likely due to additive bone marrow suppression, leading to an elevated risk of leukopenia, anemia, and thrombocytopenia, especially in patients with renal impairment or concomitant use of other myelosuppressive agents."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about AZATHIOPRINE vs ENVARSUS XR, answered by our medical review team.
AZATHIOPRINE is a Immunosuppressant that works by Azathioprine is a purine analog that inhibits purine nucleotide synthesis, thereby suppressing DNA replication and cell proliferation. It is converted to 6-mercaptopurine, which acts as a purine antagonist, inhibiting de novo purine synthesis and interfering with RNA and DNA synthesis, particularly in rapidly dividing cells such as T-lymphocytes.. ENVARSUS XR is a Calcineurin Inhibitor Immunosuppressant that works by Calcineurin inhibitor. Binds to FKBP-12, forming a complex that inhibits calcineurin phosphatase, thereby blocking T-cell activation and IL-2 transcription.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between AZATHIOPRINE and ENVARSUS XR depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of AZATHIOPRINE is: 1.5 to 2.5 mg/kg orally once daily; typical adult dose 50-150 mg/day orally. Intravenous dose is 3-5 mg/kg/day as a slow infusion over 30-60 minutes.. The standard adult dose of ENVARSUS XR is: 0.2 mg/kg/day orally once daily, with the morning meal, using extended-release tablets. Dose adjustments guided by trough concentrations.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between AZATHIOPRINE and ENVARSUS XR in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. AZATHIOPRINE is classified as Category D/X. Azathioprine is pregnancy category D. First trimester: increased risk of congenital anomalies including ventricular septal defects, limb defects, and esophageal atresia. Second and. ENVARSUS XR is classified as Category C. Envarsus XR (tacrolimus) is classified as FDA Pregnancy Category C. In the first trimester, there is an increased risk of congenital anomalies (e.g., cardiac, renal) based on anima. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.