BACKGROUND: At least 5 years of adjuvant endocrine therapy substantially reduces risks of recurrence and mortality in oestrogen-receptor positive early breast cancer. However, adherence to endocrine therapy is sub-optimal; poor adherence is associated with higher risks of recurrence and death from breast cancer, worse cancer-specific health-related quality-of-life, and increased healthcare costs. The SWEET randomised control trial aims to evaluate effectiveness and cost-effectiveness of the HT&Me intervention in reducing poor adherence to adjuvant endocrine therapy and improve cancer-specific health-related quality-of-life in women with oestrogen-receptor positive early breast cancer. METHODS: This is a UK based, pragmatic, open label randomised control trial. Participants (stages 1-3 oestrogen-receptor positive breast cancer, completed surgery, within 14 weeks of first endocrine therapy prescription; n = 1460) complete a baseline questionnaire, and are randomised to the HT&Me intervention plus usual care, or usual care alone. The HT&Me intervention is evidence-based, theory-informed and patient-centred. It consists of viewing an animation, two consultations with a SWEET study practitioner (a health care professional trained in delivering the intervention) approximately 3 months apart, access to the interactive HT&Me web-app for the 18 months, and regular monthly nudges. All participants complete follow-up questionnaires at 6, 12, and 18 months. A multi-method process evaluation will be conducted involving quantitative analysis exploring mechanisms of action of the intervention, and qualitative interviews with a sample of participants and health care professionals involved in the trial. Primary endpoints are adjuvant endocrine therapy adherence (combined self-report (Medication Adherence Report Scale) and Proportion of Days Covered calculated from prescription encashment records) and cancer-specific health-related quality-of-life (Functional Assessment of Cancer Therapy Scale- General). Secondary endpoints are adjuvant endocrine therapy-specific health-related quality-of-life and within-trial cost-utility analysis which will evaluate cost-effectiveness. DISCUSSION: The SWEET trial seeks to address a significant issue affecting the growing population of breast cancer survivors: poor adherence to adjuvant endocrine therapy. Challenges addressed and resolved within the protocol include the following: capacity at sites to deliver the intervention; variations in breast cancer services nationally; and measuring adherence. This trial has potential to improve quality of life and adherence to endocrine therapy; reducing numbers of recurrences and breast cancer deaths, benefiting women, their families and the health service. TRIAL REGISTRATION: ISRCTN Number: ISRCTN24852890 registered on 02.08.2023.