IntroductionPoor adherence to recommended vitamin and mineral supplementation after bariatric surgery is common and challenging for healthcare professionals to identify. There are several questionnaires for self-reporting of adherence to chronic medication, but none has so far been evaluated for assessment of adherence to vitamin and mineral supplementation after bariatric surgery. The aim of this study was to assess the accuracy of the 5-item Medication Adherence Report Scale (MARS-5) in measuring adherence to vitamin and mineral supplementation post bariatric surgery (gastric bypass or sleeve gastrectomy).MethodThe psychometric properties of MARS-5 for vitamin and mineral supplementation were validated in two cohorts: one at 1 year post bariatric surgery (n = 120) and the other at 2 years post-surgery (n = 211). MARS-5 was compared to pharmacy refill data for vitamin B12 and combined calcium/vitamin D as reference.ResultsCorrelation analyses demonstrated that the MARS-5 had acceptable validity compared to objectively measured adherence rates from pharmacy refill data (calculated as continuous, multiple-interval measures of medication availability/gaps-coefficient ranged from 0.49 to 0.54). Internal reliability (Cronbach's alpha) was high: 0.81 and 0.95, respectively. There was a clear ceiling effect where one out of three had a maximum score on MARS-5.ConclusionMARS-5 demonstrated acceptable psychometric properties for assessment of adherence to vitamin and mineral supplementation post bariatric surgery.