Using supermarket loyalty card data to investigate seasonal variation in laxative purchases in the UK.

While laxatives are designed to manage the symptoms of constipation, they are also known to be misused for weight management, particularly by individuals with eating disorders. This study investigates the relationship between laxative purchases and weight management by examining seasonal trends. Using real-world loyalty card data from a major UK pharmacy retailer spanning December 2013 to December 2014, we analyse self-medication purchasing patterns from 748,375 buyers to explore potential links with weight management behaviours. In pre-registered analysis, we use regression models to investigate our hypotheses: (1) the number of doses purchased would be greater in January compared to the December prior, reflecting motivations in relation to "New Year's resolutions" around weight loss, and (2) doses purchased would be greater in May-August compared to the subsequent September, reflecting an increased focus on body image during the summer. We examine differences between stimulant and non-stimulant laxatives, as stimulants are more commonly misused for weight control due to their rapid effects. To validate our findings, we compare purchasing patterns with those for weight management products over the same periods, and also include negative controls of unrelated products, including painkillers, cold and flu, hay fever, and shampoo. Our findings reveal seasonal variations in laxative purchasing, particularly for non-stimulant medications. Purchases increase in January compared to December and are higher in some summer months compared to September, which may be consistent with seasonal patterns in weight-related behaviours. Non-stimulants exhibit greater seasonal fluctuation than stimulants. Purchases of weight management products follow similar patterns, aligning with established seasonal trends in weight loss behaviours. While laxative purchase trends align with those of weight management products, these patterns provide only indirect evidence and cannot confirm underlying intent, like body image concerns. This work highlights an opportunity for loyalty card data to evaluate impacts of policy regulations in a real-world setting.