Association Between Volcanic Rock Area Ratio and Regional Variation in Acne Medication Utilisation in Japan: A Nationwide Ecological Study

Regional variation in healthcare utilisation is influenced by demographic, healthcare, and environmental factors; however, the contribution of regional geological characteristics remains poorly understood. This nationwide ecological study investigated whether the volcanic rock area ratio (VRAR) was associated with regional variation in acne medication utilisation across the 47 prefectures of Japan. Acne-specific prescription volumes for 2022 were obtained from the National Database of Health Insurance Claims and Specific Health Checkups of Japan and analysed using Spearman’s rank correlation, locally estimated scatterplot smoothing (LOESS), multivariable linear regression, influence and sensitivity analyses, and exploratory random forest modelling. A weak inverse correlation was observed between VRAR and acne-specific prescription volume per 100,000 population (ρ = −0.21, p = 0.160). LOESS suggested a possible non-linear pattern, with the lowest prescription volume occurring at intermediate VRAR values. However, quadratic regression did not provide statistical support for this pattern (p for VRAR2 = 0.494), and bootstrap analysis indicated substantial uncertainty in the estimated turning point. The direction of the correlation remained negative in all leave-one-prefecture-out analyses, and alternative assumptions regarding NDB small-count suppression produced materially unchanged results. In multivariable linear regression, VRAR remained negatively associated with prescription volume but did not reach statistical significance after covariate adjustment. These findings indicate an exploratory ecological pattern in the relationship between regional volcanic geological characteristics and acne medication utilisation but do not establish a statistically supported association, a stable non-linear relationship, or any specific environmental exposure or biological mechanism.