Economic burden and cost of non-adherence in type 2 diabetes: a systematic review of middle- and high-income countries

Type 2 diabetes mellitus (T2DM) poses a growing economic burden in middle- and high-income countries due to its high prevalence and long-term complications. This study aimed to quantify and compare the annual per-capita direct and indirect costs of T2DM in adults of middle- and high-income countries, and to examine how treatment adherence relates to complications, hospitalizations, and costs, while also assessing the completeness of economic reporting. A PRISMA-guided systematic review (January 2014—March 2024) searched MEDLINE (via PubMed), Embase, PsycINFO, and EconLit for adult studies reporting T2DM costs; reporting completeness was appraised with Consolidated Health Economic Evaluation Reporting Standards (CHEERS). All costs were standardized to annual per-person 2024 United States dollars (USD) adjusted for PPP (PPP-USD) using local Consumer Price Index (CPI) inflation and Purchasing Power Parity (PPP) conversion. When necessary, per-person values were derived from aggregated cost data. Fifteen studies met inclusion criteria. Direct costs varied widely, ranging from USD 17.74 per person-year in Pakistan to USD 623 in Spain, while indirect costs were reported in 5 of 15 (33.3%) studies and were highest in South Korea (USD 1,068) and China (USD 929.5), mainly from productivity losses and premature mortality. Higher adherence was associated with fewer complications and hospitalizations despite higher pharmacy spending, and adherence was frequently shaped by education, geographic context, and emotional distress. Beyond direct medical spending, the economic burden of T2DM is strongly influenced by indirect costs, especially productivity losses and premature mortality, which remain under-measured. Strengthening adherence and adopting standardized, societal-perspective costing with routine reporting of indirect costs, alongside near-term policy levers such as primary-care adherence support and targeted out-of-pocket relief, could improve comparability, equity, and efficiency by redirecting resources from avoidable hospitalizations toward sustained adherence support. The study protocol was registered in the International prospective register of systematic reviews PROSPERO database under registration number CRD42024519296 (Gómez-Torres et al, Int J Environ Res Public Health 20:328, 2024)