Adherence to therapy has been reported worldwide as a major problem, and that is particularly relevant on inhaled therapy for Asthma and Chronic Obstructive Pulmonary Disease (COPD), considering its barriers and features. We reviewed the global literature reporting the main determinants for adherence on these patients.

Searches were made using the Cochrane Library, MEDLINE, EMBASE and ISI Web of Science databases. Analytical, observational and epidemiological studies (cohort, case-control and cross-sectional studies) were included, reporting association between any type of determinant and the adherence for inhaler therapy on Asthma or COPD. Random-effects meta-analysis were used to summarise the numerical effect estimates.

47 studies were included, including a total of 54.765 participants. In meta-analyses, the significant determinants of adherence to inhaled therapy were: older age [RR = 1.07 (1.03–1.10); I² = 94; p < 0.0001] good disease knowledge/literacy [RR = 1.37 (1.28–1.47); I² = 14; p = 0.33]; obesity [RR = 1.30 (1.12–1.50); I² = 0; p = 0.37]; good cognitive performance [RR = 1.28 (1.17–1.40); I² = 0; p = 0.62]; higher income [RR = 1.63 (1.05–2.56); I² = 0; p = 0.52]; being employed [RR = 0.87 (0.83–0.90); I² = 0; p = 0.76] and using multiple drugs/inhalers [RR = 0.81 (0.79–0.84); I² = 0; p = 0.80]. Overall, the strength of the underlying evidence was only low to moderate.

Many determinants may be associated to patient's adherence, and personalised interventions should be taken in clinical practice to address it by gaining an understanding of their individual features.