Ola Nordqvist,1,2 Ulrika Lönnbom Svensson,3 Lars Brudin,4,5 Pär Wanby,2,6 Martin Carlsson2,7 1The Pharmaceutical Department, Region Kalmar County, Kalmar, Sweden; 2eHealth Institute, Data Intensive Sciences and Applications (DISA), Department of Medicine and Optometry, Linnaeus University, Kalmar, Sweden; 3Department of Chemistry and Biomedical Sciences, Linnaeus University, Kalmar, Sweden; 4Department of Clinical Physiology, County Hospital of Kalmar, Kalmar, Sweden; 5Department of Medical and Health Sciences, University of Linkoping, Linköping, Sweden; 6Section of Endocrinology, Department of Internal Medicine, County Hospital of Kalmar, Kalmar, Sweden; 7Department of Clinical Chemistry, County Hospital of Kalmar, Kalmar, SwedenPurpose: Several medications are known to cause vitamin D deficiency. The aim of this study is to describe vitamin D testing and supplementation in patients using these “risk medications”, thereby assessing adherence to medical guidelines.Patients and methods: A database with electronic health records for the population in a Swedish County (≈240,000 inhabitants) was screened for patients prescribed the pre-defined “risk medications” during a 2-year period (2014–2015). In total, 12,194 patients were prescribed “risk medications” pertaining to one of the three included pharmaceutical groups. Vitamin D testing and concomitant vitamin D supplementation, including differences between the included pharmaceutical groups, was explored by matching personal identification numbers. Results: Corticosteroids were prescribed to 10,003 of the patients, antiepileptic drugs to 1,101, and drugs mainly reducing vitamin D uptake to 864. Two hundred twenty-six patients were prescribed >1 “risk medication”. Seven hundred eighty-seven patients (6.5%) had been tested during the 2-year period. There were no differences regarding testing frequency between groups. Concomitant supplements were prescribed to 3,911 patients (32.1%). It was more common to be prescribed supplements when treated with corticosteroids. Vitamin D supplementation was more common among tested patients in all three groups. Women were tested and supplemented to a higher extent. The mean vitamin D level was 69 nmol/L. Vitamin D deficiency was found in 24.1% of tested patients, while 41.3% had optimal levels. It was less common to be deficient and more common to have optimal levels among patients prescribed corticosteroids. Conclusion: Adherence to medical guidelines comprising testing and supplementation of patients prescribed drugs causing vitamin D deficiency needs improvement in Sweden. Keywords: drug-induced vitamin D deficiency, medication risk management, big data, electronic health records, vitamin D, precision medicin