Risk of Infection Before and After Thiopurine Therapy in Inflammatory Bowel Disease: A Population-Based Study

To assess the incidence, type, and severity of infections in patients with IBD before and after starting thiopurine therapy.

Thiopurines have been used for decades in the treatment of inflammatory bowel disease (IBD), yet real-world data regarding the extent and types of infections associated with their use are scarce.

We included all thiopurine-naive adults with IBD who initiated thiopurine therapy between January 1, 2015, and December 31, 2022, at Skåne University Hospital, Sweden. Patients were identified through the Swedish IBD Quality Register, and medical records from primary and secondary care were reviewed. Infection incidence was recorded from 2 years before thiopurine initiation until discontinuation, switch to other therapy, or December 31, 2023, whichever came first.

Among 244 patients, the incidence rate (IR) of any infection increased from 22.4 (95% CI: 18.2-27.6) to 31.8 (95% CI: 26.7-37.9) per 100 person-years after thiopurine initiation. Thiopurine use was associated with increased risk of any infection (IRR=1.4, 95% CI: 1.2-1.7), mild infections (IRR=1.8, 95% CI: 1.2-2.5), viral infections (IRR=2.3, 95% CI: 1.6-3.3), and upper respiratory tract infections (IRR=2.4, 95% CI: 1.5-3.8). No significant increase in moderate-to-severe infections was observed (IRR=1.2, 95% CI: 0.9-1.5).

Thiopurines increased the risk of mild infections, especially viral infections, but not the risk of moderate-to-severe infections. While thiopurines appear relatively safe in terms of infection risk, the rise in mild infections remains a clinically relevant burden, potentially affecting quality of life and adherence.