Sugar-sweetened-beverage levy exposure and dental-hygiene care demand among Ghanaian adolescents: an interrupted time-series estimation of therapy-visit substitution

Ghana's 2018 excise levy on sugar-sweetened beverages was introduced with oral-health revenue earmarking, yet whether the levy shifted adolescent dental-hygiene care from curative therapy visits toward preventive hygiene attendance remains unestimated. the present analysis specifies a counterfactual framework in which the levy operates through two channels: a price-induced reduction in sugar-sweetened-beverage consumption that lowers caries incidence, and a revenue-induced expansion of community dental-hygiene capacity that lowers the time cost of preventive attendance. Using the DHS Program indicator 'Adolescents 10-19' (HC_AGEG_P_ADL) for 36 African countries over 1990-2025 (576 country-survey observations; mean 23.731, SD 1.777, range 21.28 in South Africa to 27.63 in Eswatini), the paper demonstrates that the indicator's cross-country variance is too compressed to serve as an outcome in a Ghana-only interrupted time-series design, and that the design must therefore be specified as a comparative computed-control estimation with Ghana as the treated unit and the 35-country pool as the donor pool. The paper derives the identifying assumptions, the substitution-versus-prevention decomposition, and the minimum detectable effect that the design can credibly recover. It argues that therapy-visit substitution is the wrong primary estimand: the levy's dental-hygiene consequence is a composition shift within a roughly fixed visit volume, and the estimand should be the ratio of preventive to curative visits rather than total visit counts.