Azathioprine (AZA) is a purine analogue that is administered primarily by mouth and has an inhibitory effect on lymphocyte and monocyte proliferation, antibody production, natural killer cell activity, and cellular and humoral immune responses. The main adverse reactions are nausea, vomiting, abdominal pain, oral ulcers, bone marrow suppression including leukopenia or thrombocytopenia and anemia; others can be seen as rash, elevated liver enzymes or blood uric acid, alopecia, peripheral neuritis, proteinuria and secondary infections; severe hypersensitivity reactions are rare but serious, including fever, chills, acute interstitial nephritis, hepatitis, etc. The most common adverse reactions in the Chinese population are leukopenia and liver damage. The most common adverse reactions in the Chinese population are leukopenia and liver damage. Both domestic and international consensus opinions confirm that AZA is an important drug for maintaining clinical remission of Crohn’s disease, however, there is no consensus opinion on the dose of AZA for CD treatment in China. The consensus in Western countries recommends a dose of 1.5 to 2.5 mg/Kg/d. Is this dose suitable for Asian populations? Japanese studies have reported that 1mg/Kg/d is sufficient, but it is a non-controlled study. Few domestic studies have been conducted on the dose of AZA for CD treatment. Our previous open study of AZA for CD reported the efficacy and safety of 2mg/Kg/d dose. However, at a dose of 2mg/Kg/d, some patients experienced adverse effects and some patients again experienced poor disease control. This indicates that the metabolism of azathioprine is different in each individual, and the same dose of the drug may be metabolized slowly in patient A, which may lead to excessive drug concentration and side effects, and rapidly in patient B, which may lead to low drug concentration and reduced drug efficacy. 75% of the patients with azathioprine failure had a problem of insufficient drug concentration. At the same time, the metabolism of AZA in the body is a complex process, and most studies have concluded that the concentration of 6-TGN, the active metabolite of azathioprine, is associated with the clinical efficacy and adverse effects of Crohn’s disease, with higher concentrations of 6-TGN in erythrocytes leading to clinical remission and higher concentrations of 6-TGN in erythrocytes leading to clinical remission in those who are still clinically active, but the occurrence of adverse effects may also However, the incidence of adverse effects may also increase accordingly. The positive predictive value for clinical remission was 85.7% when the intraerythrocytic 6-TGN concentration was >292 p mol/8×108RBC. 6-MMP is another metabolite of AZA, and its concentration is closely related to the adverse effects of the drug, especially liver function impairment. In clinical practice, we can increase the concentration of 6-TGN by competitively inhibiting 6-MMP production with the addition of allopurinol. Azathioprine can be administered orally, is inexpensive, has relatively few side effects, and is the first-line agent for Crohn’s disease. Monitoring drug concentrations (6-TG, 6-MMP) during the application of azathioprine can effectively reduce drug side effects and enhance drug efficacy, which is important for us to optimize the therapeutic regimen of azathioprine.