Declining antibiotic consumption and changing sexually transmitted infection dynamics in Japan: a national ecological time-series analysis, 2013–2024

OBJECTIVES: We examined whether declining antibiotic consumption in Japan coincided with changes in sexually transmitted infection (STI) indicators. METHODS: We conducted a national ecological time-series analysis using annual data from 2013 to 2024. Antibiotic consumption was measured as defined daily doses per 1,000 inhabitants per day. Outcomes were syphilis from all-case surveillance and chlamydia and gonorrhea from sentinel surveillance. Log-linear regression models with appropriate offsets estimated incidence rate ratios (IRRs) per 10% increase in antibiotic consumption. Sensitivity analyses excluded the COVID-19 pandemic period. RESULTS: Overall antibiotic consumption declined substantially during the study period, whereas syphilis notifications increased markedly. Chlamydia increased gradually, and gonorrhea fluctuated. In primary analyses, a 10% increase in antibiotic consumption was associated with lower incidence of chlamydia (IRR 0.964, 95% CI 0.949-0.978) and gonorrhea (0.836, 0.758-0.922). The association with syphilis was imprecise and not statistically significant (0.836, 0.643-1.087). Sensitivity analyses excluding 2020-2022 yielded similar directional findings. CONCLUSIONS: Declining antibiotic consumption coincided with changes in some STI indicators in Japan, but associations were heterogeneous across infections. These ecological findings are hypothesis-generating and require further evaluation using more granular data.