Vitamin D Supplementation and Its Interaction with Common Medications: Impact on Serum Levels and Quality of Life in Adults with Comorbidities

<b>Background/Objectives</b>: Vitamin D deficiency is highly prevalent worldwide and is associated with multiple comorbidities and pharmacological treatments that may interfere with its metabolism. Evidence on the effect of supplementation across different drug user groups remains limited. <b>Methods</b>: A prospective study was conducted across community pharmacies over twelve months. Baseline socio-demographic, serum 25(OH)D concentration, quality of life (QoL), lifestyle habits, and medication use were collected. Participants received vitamin D supplementation for 12 months. Changes in vitamin D status and QoL were analyzed according to medication use. Logistic regression identified predictors of achieving adequate serum vitamin D levels (>30 ng/mL). Statistical significance was set at <i>p</i> < 0.05. <b>Results</b>: At baseline, 87.2% of 210 participants had insufficient or deficient vitamin D levels. After supplementation, mean serum vitamin D increased significantly from 21.3 ± 8.2 to 32.1 ± 12.6 ng/mL (<i>p</i> < 0.001), and QoL scores improved from 68.6 ± 18.7 to 77.8 ± 18.5 (<i>p</i> < 0.001). Dietary intake of vitamin D-rich foods and outdoor activity also increased. Supplementation improved vitamin D status among users of benzodiazepines, proton pump inhibitors, beta-blockers, statins, levothyroxine, metformin, and angiotensin-converting enzyme inhibitors, but not among corticosteroid, nonsteroidal anti-inflammatory drugs, or vitamin K antagonist. Multivariate analysis confirmed adherence as a strongest predictor of vitamin D adequacy (OR = 15.31, 95% CI = 2.90-80.75), while corticosteroid therapy, diabetes, and hypercholesterolemia were negatively associated. <b>Conclusions</b>: Vitamin D supplementation effectively corrected deficiency and improved QoL, but its efficacy varied according to comorbidities and medication use. Personalized supplementation strategies, emphasizing adherence and considering pharmacological profiles, may optimize outcomes. Further studies should explore mechanistic drug-nutrient interactions and long-term clinical implications.