A total of 5746 case patients had received a 6 or more months’ supply of B12 supplements; among these patients, 1962 had neither a recorded test result for B12 deficiency nor a recorded diagnosis of B12 deficiency. Similar positive associations were found using different case definitions, such as having both a low serum B12 level and a prescription for vitamin B12 supplements (OR, 1.50 [95% CI, 1.26-1.78]). Vitamin B12 deficiency was also associated with a 2 or more years’ supply of H2RAs (OR, 1.25 [95% CI, 1.17-1.34]).