Tobacco smoking is a leading cause of death globally, resulting in over 7 million deaths annually. While smoking prevalence has declined in many high-income countries, the absolute number of deaths continues to rise in lower- and middle-income countries (LMICs) due to population growth and the delayed onset of smoking-related diseases. The World Health Organization's Framework Convention for Tobacco Control (FCTC), adopted in 2003 and legally binding in 2005, is a key global strategy to reduce smoking. The FCTC includes comprehensive measures such as demand reduction, countering smuggling, protection from secondhand smoke, and regulation of tobacco product content, packaging, advertising, and sponsorship. The effectiveness of the FCTC, particularly in LMICs, remains debated, with previous evaluations often relying on assumptions or limited data. This study aims to rigorously assess the FCTC's impact by employing interrupted time series analysis (ITSA), a method well-suited for evaluating interventions where randomization is not feasible.

Existing research on the FCTC's effectiveness presents mixed results. Some studies, employing methods with several limitations such as reliance on limited assumptions or focusing solely on MPOWER coverage, have cast doubt on its impact. The variability in surveys used, changing income levels, and concurrent implementation of various tobacco control policies hinder the use of typical regression analyses to effectively evaluate FCTC's role and quantify the impact of taxation. These limitations highlight the need for a robust analytical approach like ITSA to isolate the FCTC's effect from other factors.

The study used data from the Global Burden of Disease (GBD) study, which compiled data from 3,625 nationally representative surveys on tobacco use from 1990 to 2020. The researchers analyzed data for 171 countries, excluding China, representing 89% of the global population in 2021. They employed interrupted time series analysis (ITSA) to compare trends in the number and prevalence of smokers below age 25 (where smoking initiation is most prevalent and response to interventions is greatest) and the quit ratio (former to ever smokers) among adults aged 45-59 (when most quitting occurs). The analyses accounted for the non-linearity of smoking prevalence trends and incorporated variation in FCTC ratification years and the concurrent use of excise taxes to raise tobacco prices. Five ITSA models were tested to accommodate varying degrees of nonlinearity in pre- and post-ratification trends. Model selection was guided by the Akaike information criterion (AIC). Ordinary least squares regression with Newey-West correction for autocorrelation and heteroskedasticity was utilized. Countries were grouped by World Bank income levels and classified as 'high-tax change' (tax burden increased by ≥10 percentage points concurrently with FCTC ratification) or 'low-tax change'. The study projected pre-FCTC trends to estimate the reduction in the number of smokers after ratification and compared the actual number to the projected values. Sensitivity analyses were performed to assess the robustness of the findings, considering varying lengths of pre-ratification periods and the inclusion of only high-quality survey data (more than 10 surveys per country from 1990 to 2020).

After FCTC ratification, the prevalence of current smokers below age 25 decreased at an average annual rate of -0.8% (95% CI = -1.0 to -0.5) compared to the pre-FCTC period. The quit ratio at ages 45-59 increased at an annual average of 0.1% (95% CI = 0.1 to 0.2). Countries with high-tax changes experienced steeper decreases in prevalence (-2.1%, 95% CI = -2.8 to -1.4) and greater increases in the quit ratio (0.5%, 95% CI = 0.4 to 0.6) compared to low-tax change countries. Ten years after ratification, there were an estimated 24 million fewer young smokers (15.5% reduction) and 2 million more quitters globally. If all countries had implemented tax increases similar to high-tax change countries, the reduction would have been even greater (44 million fewer young smokers and 5.2 million more quitters). China showed a paradoxical increase in smoking prevalence post-FCTC, likely due to the state-owned nature of its tobacco industry and the availability of various price points and brands. The sharpest declines in prevalence post-FCTC occurred in LICs.

This study provides robust evidence demonstrating the FCTC's significant role in reducing smoking prevalence, particularly in conjunction with substantial tax increases. The findings highlight the effectiveness of the FCTC in accelerating declines in smoking initiation and promoting cessation. The stronger effects in countries with higher tax increases support the established evidence that large excise taxes are the most effective intervention. While the study doesn't definitively prove that taxation is the sole driver of the observed changes, it strongly suggests a synergistic effect between the FCTC and increased taxation. The contrasting outcome in China underscores the unique challenges posed by state-controlled tobacco industries. The study's findings have crucial policy implications, emphasizing the need for countries to ratify the FCTC, implement substantial tobacco tax increases, and utilize other non-price interventions to curb smoking.

The FCTC, particularly when combined with substantial tax increases, has demonstrably reduced smoking prevalence and increased cessation rates globally. The study's findings reinforce the importance of comprehensive tobacco control strategies, including strong tobacco taxation, to reduce the substantial health and economic burden of smoking. Future research could explore the impact of specific non-price interventions on smoking behaviors in different contexts and investigate the mechanisms underlying the observed country-level variations in effectiveness.

The study's limitations include the aggregation of male and female smokers in the analyses, limiting the exploration of sex-specific differences in cessation. The grouping of countries by income level and tax increase, instead of analyzing individual countries, prevents the consideration of country-level covariates that might affect smoking trends. The use of the GBD data relies on complex estimation and modelling techniques which might potentially introduce smoothing or other limitations into data analysis. However, sensitivity analyses using high-quality survey data yielded similar results, suggesting that these limitations did not significantly bias the main findings.