Personal Formularies of Primary Care Physicians Across 4 Health Care Systems

IMPORTANCE: More conservative prescribing has the potential to reduce adverse drug events and patient harm and cost; however, no method exists defining the extent to which individual clinicians prescribe conservatively. One potential domain is prescribing a more limited number of drugs. Personal formularies—defined as the number and mix of unique, newly initiated drugs prescribed by a physician—may enable comparisons among clinicians, practices, and institutions. OBJECTIVES: To develop a method of defining primary care physicians’ personal formularies and examine how they differ among primary care physicians at 4 institutions; evaluate associations between personal formularies and patient, physician, and practice site characteristics; and empirically derive and examine the variability of the top 200 core drugs prescribed at the 4 sites. DESIGN, SETTING, AND PARTICIPANTS: This retrospective cohort study was conducted at 4 US health care systems among 4655 internal and family medicine physicians and 4 930 707 patients who had at least 1 visit to these physicians between January 1, 2017, and December 31, 2018. EXPOSURES: Personal formulary size was defined as the number of unique, newly initiated drugs. MAIN OUTCOMES AND MEASURES: Personal formulary size and drugs used, physician and patient characteristics, core drugs, and analysis of selected drug classes. RESULTS: The study population included 4655 primary care physicians (2274 women [48.9%]; mean [SD] age, 48.5 [4.4] years) and 4 930 707 patients (16.5% women; mean [SD] age, 51.9 [8.3] years). There were 41 378 903 outpatient prescriptions written, of which 9 496 766 (23.0%) were new starts. Institution median personal formulary size ranged from 150 (interquartile range, 82.0-212.0) to 296 (interquartile range, 230.0-347.0) drugs. In multivariable modeling, personal formulary size was significantly associated with panel size (total number of unique patients with face-to-face encounters during the study period; 1.2 medications per 100 patients), physician’s total number of encounters (5.7 drugs per 10% increase), and physician’s sex (−6.2 drugs per 100 patients for female physicians). There were 1527 unique, newly prescribed drugs across the 4 sites. Fewer than half the drugs (626 [41.0%]) were used at every site. Physicians’ prescribing of drugs from a pooled core list varied from 0% to 100% of their prescriptions. CONCLUSIONS AND RELEVANCE: Personal formularies, measured at the level of individual physicians and institutions, reveal variability in size and mix of drugs. Similarly, defining a list of commonly prescribed core drugs in primary care revealed interphysician and interinstitutional differences. Personal formularies and core medication lists enable comparisons and may identify outliers and opportunities for safer and more appropriate prescribing.