Risk of complications from specific classes of drugs for inflammatory bowel diseases (IBD) can be kept low by respecting contra-indications. Patients with IBD frequently develop serious infections, due to the disease itself or its treatment. At the time of diagnosis, patients’ vaccination calendars should be updated according to IBD guidelines-live vaccines should be postponed for patients receiving immunosuppressive drugs. Opportunistic infections should be detected and the vaccine against Pneumococcus should be given before patients begin immunosuppressive therapy. Thiopurines promote serious viral infections, in particular, whereas tumor necrosis factor (TNF) antagonists promote all types of serious and opportunistic infections. Severe forms of varicella can be prevented by vaccinating seronegative patients against varicella zoster virus. Detection and treatment of latent tuberculosis is mandatory before starting anti-TNF therapy and other new IBD drugs. Tofacitinib promotes herpes zoster infection in a dose- and age-dependent manner. Physicians should consider giving patients live vaccines against herpes zoster before they begin immunosuppressive therapy or a recombinant vaccine, when available, at any time point during treatment. Risk of thiopurine-induced lymphomas can be lowered by limiting the use of thiopurines in patients who are seronegative for Epstein-Barr virus (especially young men) and in older men. The risk of lymphoma related to monotherapy with anti-TNF agents is still unclear. There are no robust data on carcinogenic effects of recently developed IBD drugs. For patients with previous cancer at substantial risk of recurrence, physicians should try to implement a pause in the use of immunosuppressive therapy (except in patients with severe disease and no therapeutic alternative) and prioritize use of IBD drugs with lowest carcinogenic effects. Finally, sun protection and skin surveillance from the time of diagnosis are recommended.Copyright © 2020 AGA Institute. Published by Elsevier Inc. All rights reserved.