Lifelong vitamin and mineral supplementation is recommended after bariatric surgery to prevent nutritional deficiencies, but it is not clear to which extent patients adhere to the recommended medication. The aim of this study was to assess adherence to supplementation. Additionally, we assessed predictors of low adherence and biochemical deficiencies.

263 adults were followed two years post bariatric surgery (89.4 % Roux-en-Y gastric bypass, 10.6 % sleeve gastrectomy) regarding adherence to calcium-vitamin D, vitamin B₁₂ and iron. Pharmacy refill data was used to objectively capture adherence-rate. Predictors of low adherence were assessed via questionnaires and medical records. Blood sampling was performed preoperatively, and 1- and 2-years post-surgery.

About 60 % of the study participants had high adherence-rate (≥ 80 %) two years post-surgery, but a significant proportion had discontinued the intended lifelong supplementation: 15 % for calcium-vitamin D and 12 % for B₁₂. Biochemical deficiencies were uncommon two years after surgery, both in adherent and non-persistent subjects, except for a 10 % prevalence of anemia. Younger age, experience of side effects, mental health problems and no regular use of medication before surgery, were factors associated with low adherence.

Substantial variation existed in adherence to supplementation regimens after bariatric surgery during the first two years after bariatric surgery, but biochemical deficiencies were rare. The results indicate need for tailored follow-up and strategies to support long-term persistence.