The strengths of this study include its large size, access to care for all members, up to 15 years of exposure data, the as- certainment of all recorded diagnoses of vitamin B12 defi- ciency arising within the study population (thereby minimiz- ing referral bias), detailed electronic data for dispensed medication (eliminating recall bias), and the use of a control group that approximates the underlying general population of the region. The large study size permitted evaluation for small