The safety profiles of IV and SC formulations of trastuzumab have been individually studied in separate clinical trials.23,27,28 In Phase 2 and Phase 3 studies evaluating IV trastuzumab in patients with HER2-positive breast cancer, the most commonly reported adverse events (AEs) were infections, headache, nausea, fever and chills.4,23,27 In the SafeHer trial, a two-cohort, nonrandomised open-label study of the overall safety of SC trastuzumab in combination with chemotherapy in patients with HER2-positive breast cancer, the most commonly reported AEs were diarrhoea, fatigue and arthralgia.28 Here, ERBB2 is linked to breast cancer.