To the Editor: Treatment of alopecia areata universalis (AAU) is challenging, and systemic and even immunosuppressive drugs are often required. Therapy with topical immunotherapy with diphencyprone (DFCP) and oral corticosteroids are the most used treatments in adult patients with AAU. Some patients are resistant to multiple therapies.1Alkhalifah A. Alsantali A. Wang E. McElwee K.J. Shapiro J. Alopecia areata update: part II. Treatment.J Am Acad Dermatol. 2010; 62 (quiz 203-204): 191-202Abstract Full Text Full Text PDF PubMed Scopus (234) Google Scholar, 2Spano F. Donovan J.C. Alopecia areata: Part 2: treatment.Can Fam physician Médecin Fam Can. 2015; 61: 757-761Google Scholar The objective of this study was to evaluate the efficacy and safety of oral azathioprine for adult patients with recalcitrant AAU. A prospective study including adult patients with recalcitrant AAU (non-responders to oral corticosteroids and DFCP) between 2010 and 2015 was designed. Treatment with azathioprine at a dosage of 2.5 mg/kg/day (adjusted based thiopurine methyltransferase [TPMT] level) was administered. Therapeutic response was assessed as no response (no regrowth), partial response (regrowth of <75% of scalp), and complete response (regrowth of 75% or more of scalp). Persistent response was defined as the maintenance of >75% of regrowth after 6 months of the withdrawal of therapy. Overall, 14 patients (8 females [57%] and 6 males [43%]) with a mean age of 37.7 years (range, 23-59 years) were included. The mean time since onset of AAU was 24.5 months (range, 8-72 months). The mean dosage of azathioprine used was 142 mg daily. Therapeutic response was observed in 6 of 14 patients (43%), all of them achieving complete response, also including regrowth of body hair (Fig 1). There were no identifiable prognostic factors statistically associated with better response. The mean time until response was 4.7 months (range, 4-6 months), while the mean duration of therapy was 9.8 months (range, 1-36 months). Two patients (14.3%) presented with relapses after a mean time of 2.5 months after the withdrawal of azathioprine. On the other hand, persistent response was observed in 4 patients (29%), with a mean follow-up time of 18 months (range, 6-30 months). Adverse effects were detected in 5 patients (35%): diarrhea (2 patients) and elevation of liver enzymes, pancreatitis, and bone marrow suppression (1 patient each). In 4 cases (28%), treatment had to be discontinued. Azathioprine is a purine antimetabolite drug widely used in dermatologic conditions.3Wise M. Callen J.P. Azathioprine: a guide for the management of dermatology patients.Dermatol Ther. 2007; 20: 206-215Crossref Scopus (40) Google Scholar, 4Sidbury R. Davis D.M. Cohen D.E. et al.Guidelines of care for the management of atopic dermatitis: section 3. Management and treatment with phototherapy and systemic agents.J Am Acad Dermatol. 2014; 71: 327-349Abstract Full Text Full Text PDF PubMed Scopus (562) Google Scholar However, the reports of the usefulness of this drug in AA are anecdotal. Farshi et al5Farshi S. Mansouri P. Safar F. Khiabanloo S.R. Could azathioprine be considered as a therapeutic alternative in the treatment of alopecia areata? A pilot study.Int J Dermatol. 2010; 49: 1188-1193Crossref PubMed Scopus (50) Google Scholar reported a series of 20 AA patients (5 with AAU) treated with oral azathioprine. The mean global regrowth percentage was 52.3%, although response to azathioprine was assessed based on the change in the percentage of hair regrowth, so a direct comparison with our study cannot be performed. Adverse effects appeared in 4 patients (20%), which is inferior to our study. The effectiveness of azathioprine in our study was acceptable (43% of patients), taking into account that patients presented recalcitrant AAU. In addition, 2 out of 3 responders presented a persistent response. Nevertheless, the rate of adverse effects was higher than previous reports,5Farshi S. Mansouri P. Safar F. Khiabanloo S.R. Could azathioprine be considered as a therapeutic alternative in the treatment of alopecia areata? A pilot study.Int J Dermatol. 2010; 49: 1188-1193Crossref PubMed Scopus (50) Google Scholar including 2 patients with serious adverse effects: pancreatitis and bone marrow suppression, both in patients with normal TPMT. In conclusion, oral azathioprine may be an interesting therapeutic option for adult patients with recalcitrant AAU. However, adverse effects may appear, so a strict follow-up with frequent blood tests is encouraged.