Purpose: Subcutaneous trastuzumab has shown non-inferior efficacy to intravenous trastuzumab in patients with HER2 positive early breast cancer [1,2]. Furthermore, subcutaneous trastuzumab was the preferred treatment option when compared with intravenous trastuzumab by patients [3]. As a result NHS England has encouraged widespread adoption, with the majority of patients switching to the subcutaneous formulation. The trastuzumab ‘Summary of Product Characteristics’ recommends observation for 6 h following the first dose. The low rate of reactions suggests that the prolonged observation times may be unnecessary. We conducted an audit to document the rate of adverse events in association with subcutaneous trastuzumab administration in The Royal Marsden NHS Trust.