PURPOSE:The aim of this study was to assess the safety profiles of two biosimilar medicines (rituximab and trastuzumab) in the treatment of cancer patients within a Portuguese oncology hospital. METHODS:This hospital-based prospective observational study followed a cohort event monitoring approach focused on signalling suspected adverse drug reactions (ADRs). Patients undergoing treatment with rituximab biosimilar CT-P10 (Truxima®) or trastuzumab biosimilar CT-P6 (Herzuma®) were recruited over an 11-month and a 6-month period, respectively. Clinicians identified eligible patients and used paper-based forms to report all ADRs associated with biosimilar medicines. ADR case reports were assessed for seriousness, expectedness and causality in the Pharmacovigilance Unit of Coimbra. RESULTS:Ninety-four patients received biosimilar medicines (rituximab, n = 35; trastuzumab, n = 59). Of those, 4 patients (11.4%) experienced 16 ADRs with rituximab and 1 patient (1.7%) experienced 5 ADRs with trastuzumab. All case reports contained serious and expected ADRs that were at least probably related with biosimilar medicines under study. Based on the MedDRA PT coding, the most reported ADR for rituximab CT-P10 was chest discomfort (n = 4; 19.1%), followed by odynophagia (n = 2; 9.5%). Trastuzumab CT-P6 was associated with back pain, headache, pain in extremity, tachypnoea and tremor (each, n = 1; 4.8%). CONCLUSION:The results of this study suggest that using biosimilar rituximab and biosimilar trastuzumab to treat cancer patients in the real-world clinical setting is associated with acceptable safety profiles. No new safety problems were identified.