Timing of myelosuppression during thiopurine therapy for inflammatory bowel disease: implications for monitoring recommendations.

Background & Aims: Thiopurines (azathioprine and 6-mercaptopurine) can induce life-threatening myelosuppression. This study determined the frequency, timing, and outcomes of mild and severe myelosuppression after initiation of thiopurine therapy.

Methods: This retrospective cohort study included patients with inflammatory bowel disease who were new users of thiopurines; those tested for thiopurine methyltransferase levels before therapy were excluded.