RT Journal Article SR Electronic T1 Vitamin D and vitamin B12 deficiencies in patients with small intestinal carcinoid tumour: is opioid use disorder a confounding factor in the diagnosis? JF BMJ Case Reports JO BMJ Case Reports FD BMJ Publishing Group Ltd SP e227430 DO 10.1136/bcr-2018-227430 VO 12 IS 3 A1 Richard Fagan A1 Syed Sabeeh Najam Bokhari A1 Faisal Inayat YR 2019 UL http://casereports.bmj.com/content/12/3/e227430.abstract AB Carcinoid tumours have the ability to secrete various peptides and bioamines that lead to carcinoid syndrome manifested as cutaneous flushing, diarrhoea, bronchial constriction and cardiac involvement. The deficiencies of vitamins D and B12 have previously been reported in patients with carcinoid tumours presumably due to chronic diarrhoea associated with the carcinoid syndrome. Herein, we chronicle the case of a patient with opioid use disorder who presented with small bowel obstruction that was found to be caused by a midgut carcinoid tumour. Laboratory studies revealed deficiencies of vitamins D and B12 even though he denied diarrhoea and had no other aetiology of deficiencies of these vitamins. Additionally, this paper presents a review of the published medical literature pertaining to clinical features, diagnostic investigations and treatment of intestinal carcinoid tumours and explores possible explanations for the observed deficiencies in these patients.