Background Non-adherence to asthma/COPD inhalation medication is prevalent. Shared decision making (SDM) is proposed as an intervention to improve medication adherence. Despite its wide promotion, evidence of SDM?s effectiveness is scarce. Also, it is unknown to what degree patients experience SDM and how it relates to medication adherence.

Study aim To (i) assess the level of SDM (pre- and during COVID), (ii) medication adherence, (iii) explore the relation between SDM and medication adherence and (iv) investigate possible underlying mechanisms.

Methods Cross-sectional observational study. A survey was distributed among Dutch patients with COPD and/or asthma using inhaled medication. Medication adherence was measured using the Test of Adherence to Inhalers-10, and SDM by the SDM-Q9. Feeling of competence, relatedness and feeling of autonomy from the Self-Determination Theory were considered as possible mechanisms. The primary outcome was non-adherence (TAI<50). Regression analyses assessed associations of non-adherence with SDM.

Results 396 patients (mean age: 63.1, 55.8% female, 51.0% asthma, 49.0% COPD +/- asthma) were included. Mean SDM score was 26.7 (SD 12.1, range 0-45) and non-adherence was 61.1%. The odds ratio for the association of SDM score with non-adherence was 1.01 (95%CI: 0.99-1.02). This changed minimal when adjusted for the suggested mediators (mediating effect <3%).

Conclusion SDM in asthma/COPD is suboptimal (both pre and during COVID). No association between SDM and medication adherence was observed. Factors related to feeling of competence, relatedness and feeling of autonomy did not meaningfully explain this finding.