Abstract

Introduction
Some cancers are diagnosed late, making them harder to treat. People with an undiagnosed cancer may use over-the-counter medications to manage non-specific cancer-related symptoms that often mimic other more common, easily treatable conditions. Results from the original Cancer Loyalty Card Study (CLOCS) suggest there may be an increase in purchases of pain and indigestion medication 8–9 months before an ovarian cancer diagnosis. We aim to validate the CLOCS findings by exploring whether a significant change in medication purchases could be an indication for early signs of the following cancer types: oesophageal, stomach (gastric), colorectal (bowel), pancreatic, liver, bladder, endometrial, uterine sarcoma, ovarian and vulval, using data collected through store loyalty cards.

Methods and analysis
Using a retrospective case-control design, we aim to recruit 1450 participants with one of the cancers of interest (cases) and 1450 participants without cancer (controls) in the UK who (or whose household members) hold a loyalty card with at least one participating high street retailer. We will use pre-existing loyalty card data to compare past purchase patterns of cases with those of controls. To assess cancer risk in participants and their purchasing patterns, we will collect information on demographic characteristics, health risk factors, lifestyle habits and behaviours, family history of cancer and any symptoms experienced prior to diagnosis (cases) and in the last year prior to study recruitment (controls). In addition, cases will be asked about their cancer diagnosis.

Ethics and dissemination
CLOCS-2 was reviewed and approved by the East Midlands-Leicester South Research Ethics Committee (23/EM/0224). Study outcomes will be disseminated through peer-reviewed publications, conferences, presentations to the research communities as well as patients and the public, the study website and other social media outlets.

Trial registration number
NCT06447064 , CPMS58679; pre-results.