Determinants of adherence to calcium/vitamin D supplements in a cross-sectional study

Adherence to combined calcium and vitamin D supplements is often suboptimal, yet the independent role of formulation attributes (pharmaceutical form, organoleptic profile, acceptability) remains understudied. The aim was to estimate Calcium/Vitamin D adherence in real-world practice and to identify associated factors, with particular emphasis on formulation characteristics and patient-perceived acceptability.

Population-based analytical cross-sectional study (STROBE guidelines). Random telephone interviews were conducted with 360 participants from a tertiary hospital catchment area in Madrid, with active prescriptions for over 6 months, complementing the data with electronic dispensing records. Global adherence was defined as the simultaneous fulfillment of: self-reported adherence >90%, a Morisky-Green result compatible with adherence, and ≥90% of expected dispensations in the previous 6 months. Formulations were classified a priori into five organoleptic groups based on their sweetener and flavor profile. Acceptability was assessed using 1-5 Likert scales for taste, texture, and overall rating. Univariate and multivariable logistic regression were used to identify factors associated with adherence.

Twenty-two different formulations were identified. Global adherence was 34.7% (95% CI: 30.0-39.8). Agreement between measures was moderate (κ = 0.38-0.60). Self-reporting (70.8%) significantly overestimated adherence compared to dispensing records (40.3%). Chewable tablets showed higher adherence than orodispersible and effervescent forms. In the multivariable model, adherence was independently associated with female sex (aOR 2.69; 95% CI: 1.59-4.59), polypharmacy (>10 drugs) (aOR 2.32; 95% CI: 1.32-4.07), polyol-based organoleptic group (aOR 2.43; 95% CI: 1.26-4.77) and overall rating of 5/5 (aOR 6.17; 95% CI: 3.16-12.50). Adverse reactions (reported by 15.3% of participants) reduced the probability of adherence (aOR 0.24; 95% CI: 0.08-0.58). Forgetfulness was the main reason for non-adherence (63.8%), followed by organoleptic problems and intolerance. The model showed good discrimination (Area under the curve 0.83).

Strict adherence to calcium/vitamin D is low, and self-reporting substantially overestimates actual compliance. Beyond classical patient-level factors, formulation tolerability and acceptability emerge as independent, potentially modifiable determinants of adherence. A simple screening strategy based on patient overall rating, adverse reactions and formulation organoleptic profile could guide early, pragmatic interventions such as adherence support and timely formulation switching.