Keywords: 6-MP; 6-mercaptopurine; ACG; ADA; AZA; American College of Gastroenterology; BSG; British Society of Gastroenterology; CD; CDAI; CI; CZP; Crohn's Disease Activity Index; Crohn's disease; ECCO; European Crohn's and Colitis Organisation; GRADE; Grading of Recommendations Assessment, Development and Evaluation; HR; IBD; IFX; MTX; OR; PICO; RCT; RR; SIR; TNF; adalimumab; azathioprine; certolizumab pegol; confidence interval; hazard ratio; inflammatory bowel disease; infliximab; methotrexate; odds ratio; population, intervention, comparator, and outcome; randomized controlled trial; relative risk; standardized incidence ratio; tumor necrosis factor.