Assigning drug exposure is a necessary first step in examining bisphosphonate (BP) treatment in observational studies using pharmacy data. To determine whether the choice of adherence level using the proportion of days covered (PDC) affected BP exposure assignment. 10,381 female health plan members who initiated oral BP therapy between 2002 and 2010 and had received 5 consecutive years of treatment were identified and subsequently followed up to 5 additional years. In each 90-day interval of follow-up, a woman was considered “on treatment” if she received the drug for more than a predetermined PDC based on pharmacy days supply and “off treatment” if she received the drug for less than that PDC. Women who continued on therapy above the PDC threshold during follow-up were considered continuously on therapy. Women who were off treatment during the first 90-days of follow-up were classified as off therapy and were followed to determine if they remained continuously off treatment. This study evaluated the extent to which varying the PDC threshold (≥ 0.5, ≥ 0.6, and ≥ 0.7) affected the proportion of women classified as “continuously on” or “continuously off” BP during follow-up. Under PDC thresholds of 0.5, 0.6, and 0.7, 48%, 43%, and 36% of women who remained on follow-up were categorized as continuously on treatment at year 2 of follow-up, and 18%, 14%, and 12% were categorized as continuously on treatment by the end of follow-up. Using these same PDC thresholds, 9%, 12%, and 15% of women were categorized as off therapy during the first quarter of follow-up and were highly likely to remain off therapy: 4%, 5%, and 5% were classified as continuously off therapy at year 2, and 4% of women were classified as such by the end of follow-up for all 3 thresholds. A PDC of 0.6 was chosen as a practical threshold for drug adherence. Varying the PDC to 0.5 or 0.7 resulted in modest changes in the proportions of women considered continuously on BP therapy. This study was supported by a grant from the National Institute of Aging and National Institute of Arthritis, Musculoskeletal and Skin Diseases at the National Institutes of Health (R01AG047230, S1). Lo has received previous research funding from Amgen and Sanofi, outside of the current study. Chandra has received previous research funding from Amgen outside of the current study. Adams has received previous research funding from Merck, Amgen, Otsuka, and Radius Health, outside of the current study. Ott previously attended a scientific advisory meeting for Amgen but declined the honorarium. Ettinger previously served as an expert witness for Teva Pharmaceuticals.