Azathioprine was administered to 33 patients (60.0%) in the wild-type group (n = 55) at a mean maximum dosage of 61.36 mg/day. As expected, heterozygous patients received azathioprine less frequently (42.9%) and at a reduced mean maximum dosage of 50.0 mg/day. 2 Strikingly, four out of five homozygous (T/T) patients were prescribed azathioprine without prior NUDT15 testing, resulting in anagen effluvium and subsequent consultations in our department because of hair loss concerns. All four patients were administered azathioprine to treat Behc ßet's disease. Discontinuation of azathioprine usually mitigated hair loss, though recovery is extended. Among those experiencing anagen effluvium, three exhibited severe leukopenia (grade 3-4) and two also manifested neutropenia. However, these patients did not exhibit nonhematologic side effects such as elevated liver enzymes. Thus, anagen effluvium can signal potential leukocytopenia in T/T allele carriers undergoing azathioprine treatment.Our study underscores the imperative need for proactive assessment of NUDT15 variants before initiating thiopurine therapy, especially in Eastern populations. Importantly, patients experiencing anagen effluvium from thiopurine prescriptions consistently displayed abrupt hair loss within 2-3 weeks, occasionally before the manifestation of leukopenia. The crucial role of dermatologists is emphasized in diagnosing anagen effluvium in individuals receiving azathioprine, assessing for concurrent NUDT15 variants, and differentiating it from other diffuse alopecia types like telogen effluvium (Table 2), where discontinuation of thiopurines is not warranted.