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Introduction The antibody-drug conjugate trastuzumab deruxtecan (T-DXd) has led to a change in the clinical management of breast cancer. Nausea and vomiting are the most common adverse events of T-DXd, which cannot be completely alleviated by standard prophylactic regimens. Olanzapine is particularly effective in preventing delayed nausea caused by chemotherapy. In this study, we will evaluate the efficacy of olanzapine in managing persistent nausea and vomiting during T-DXd treatment.Methods and analysis The ERICA study is a multicentre, placebo-controlled, double-blind, randomised phase II study with the aim to evaluate the antiemetic effects of the prophylactic olanzapine (5 mg orally, on days 1–6) or placebo combined with a 1,5-hydroxytryptamine-3 (5-HT3)–receptor antagonist and dexamethasone in patients with human epidermal growth factor receptor 2-positive metastatic breast cancer undergoing T-DXd treatment. For a period of 22 days from the day of T-DXd treatment, patients will document their experience in an electronic symptom diary daily during observational periods. The primary endpoint is the complete response rate, defined as no vomiting and no rescue medications during the ‘delayed phase’ of 24–120 hours post-T-DXd administration. In addition, we define 120–504 hour as the ‘persistent phase’ and 0–504 hours as the ‘overall phase’ for secondary endpoint analysis. We have estimated that a total sample size of at least 156 patients is needed to allow a power of 80% at a one-sided significance level of 20% in this study. The target sample size is set to 166 to account for possible case exclusions.Ethics and dissemination The study protocol is approved by the West Japan Oncology Group protocol review committee and the SHOWA University Clinical Research Review Board. The study results will be presented at international conferences and published in a peer-reviewed journal.Trial registration number jRCTs031210410.