Infliximab (IFX) combined with azathioprine (AZA) is more effective than IFX monotherapy in inflammatory bowel disease (IBD).

To identify the AZA optimal dose that is required for efficacy when receiving combination therapy.

Patients with IBD in durable remission on combination therapy were enrolled in a 1‐year, open‐label, prospective trial after randomisation into three groups: AZA steady (2‐2.5 mg/kg/day, n=28) vs AZA down (dose was halved 1‐1.25 mg/kg/day, n=27) vs AZA stopped (n=26). Primary endpoint was failure defined as occurrence of a clinical relapse and/or any change in IBD therapy.

Eighty‐one patients were included. Five (17.9%), 3 (11.1%), and 8 (30.8%) patients experienced failure at 1 year in groups AZA steady, AZA down and AZA stopped, respectively (P=.1 across the groups). The median trough levels of IFX at inclusion were close to those measured at the end of follow‐up in group AZA steady (3.65 vs 3.45 μg/mL, P=.9) and in group AZA down (3.95 vs 3.60 μg/mL, P=.5), whereas these levels dropped from 4.25 to 2.15 μg/mL (P=.02) in group AZA stopped. Four (14.3%), four (14.8%) and 11 (42.3%) patients experienced an unfavourable evolution of IFX pharmacokinetics in groups AZA steady, AZA down and AZA stopped, respectively. A threshold of 6‐TGN <105 pmoles/8.10⁸ RBC was associated with an unfavourable evolution of IFX pharmacokinetics.

Under combination therapy, AZA dose reduction, but not withdrawal, appears to be as effective as continuation of AZA at full dose.