To the Editor An important consideration in conducting drug studies is the correct definition of the variables representing the use of a drug. Mr Lam and colleagues1 defined the primary exposure as a variable (days supplied) “that combined the number of pills dispensed with the instructions for use.” They measured adherence and the dose intensity using “mean daily dose” (dispensed pills divided by exposure duration).
Gonzalez-Gonzalez C, Figueiras A. Gastric Acid–Inhibiting Medications and Vitamin B12 Deficiency. JAMA. 2014;311(14):1445. doi:10.1001/jama.2014.2098