Symptoms of ovarian cancer are non-specific and there is a low level of awareness amongst women of what they are. The most common symptoms include abdominal distension or “bloating,” feeling full or loss of appetite, pelvic or abdominal pain, increased urinary urgency or frequency, unexplained weight loss, fatigue, or changes in bowel habits. We hypothesised that women would be treating their symptoms with over-the-counter medications prior to presenting to the GP. The Cancer Loyalty Card Study (CLOCS) is an observational case-control study developed to test this hypothesis. The main results from CLOCS show that purchases of pain and indigestion medication were detected up to 8 months prior to diagnosis for ovarian cancer patients compared with controls. This occurs well in advance of the patient’s self-reported first GP visit approximately 3.5 months prior to diagnosis. These results indicate a window of opportunity to facilitate earlier presentation among those who self-care for symptoms using this novel data source and could improve ovarian cancer patients’ options for treatment and improve survival. We will also report results from a survey testing the acceptability of donating data to research, and investigations into novel ovarian cancer risk factors using this cohort. Together, these results demonstrate the value of linking health outcomes with every day data, such as transactional data, monitoring what people buy over-the-counter.