ObjectiveThere has been an increase in testing of vitamins in patients in general practice, often based on irrational indications or for non-specific symptoms, causing increasing healthcare expenditures and medicalisation of patients. So far, there is little evidence of effective strategies to reduce this overtesting in general practice. Therefore, the aim of this qualitative study was to explore the barriers and facilitators for reducing the number of (unnecessary) vitamin D and B12 laboratory tests ordered.Design and settingThis qualitative study, based on a grounded theory design, used semistructured interviews among general practitioners (GPs) and patients from two primary care networks (147 GPs, 195 000 patients). These networks participated in the Reducing Vitamin Testing in Primary Care Practice (REVERT) study, a clustered randomized trial comparing two de-implementation strategies to reduce test ordering in primary care in the Netherlands.ParticipantsTwenty-one GPs, with a maximum of 1 GP per practice who took part in the REVERT study, and 22 patients (who were invited by their GP during vitamin-related consultations) were recruited, from which 20 GPs and 19 patients agreed to participate in this study.ResultsThe most important factor hampering vitamin-test reduction programmes is the mismatch between patients and medical professionals regarding the presumed appropriate indications for testing for vitamin D and B12. In contrast, the most important facilitator for vitamin-test reduction may be updating GPs’ knowledge about test indications and their awareness of their own testing behaviour.ConclusionTo achieve a sustainable reduction in vitamin testing, guidelines with clear and uniform recommendations on evidence-based indications for vitamin testing, combined with regular (individual) feedback on test-ordering behaviour, are needed. Moreover, the general public needs access to clear and reliable information on vitamin testing. Further research is required to measure the effect of these strategies on the number of vitamin test requests.Trial registration numberWAG/mb/16/039555.