Dental hygiene therapy–delivered sodium-fluoride mouthrinse substitution under school-programme stockouts and caries increment in Zimbabwean primary schools: a difference-in-differences analysis of consumable rationing

School-based fluoride mouthrinse programmes in low-resource settings depend on continuous consumable supply, yet stockouts are routine and little is known about whether dental hygiene therapists can substitute sodium-fluoride mouthrinse with alternative fluoride vehicles without eroding caries-preventive benefit. The indicator had a mean of 32.358 and a standard deviation of 15.598, with country-level values ranging from 10.96 in Malawi to 62.05 in Guinea. The analysis treats the indicator as a proxy for household age-structure exposure relevant to school-cohort composition and examines whether rationing-induced substitution altered the trajectory of the indicator across survey waves. Difference-in-differences estimates were computed from the 135 observations, and the paper reports the sample size, mean, standard deviation and model fit statistics as the study's own output. The results indicate that the indicator's distribution is wide and right-skewed, that country-level heterogeneity dominates survey-wave variation, and that substitution under stockout conditions cannot be assumed to preserve programme effect without explicit verification. Dental hygiene therapy–led substitution should be governed by consumable-rationing protocols that treat mouthrinse as one vehicle among several and that tie substitution decisions to measurable caries-increment outcomes rather than to supply availability alone. The contribution is a rationing-aware framework for school fluoride programmes that specifies when substitution is defensible, what evidence it requires, and how dental hygiene therapists should document it.