Effect of dosing frequency on bisphosphonate medication adherence in a large longitudinal cohort of women

OBJECTIVE: To compare medication adherence with daily vs weekly bisphosphonate dosing for the treatment or prevention of osteoporosis in a broad US retail pharmacy database population. PATIENTS AND METHODS: From October 2002 through September 2003, the medication possession ratio (MPR = days of supply/ 365 days) was used to assess medication adherence. The MPR was calculated by use of daily and weekly bisphosphonate doses in a longitudinal cohort of patients who received prescriptions from 14,000 US retail pharmacies. Patient prescription information was from a database accessed through NDCHealth in Atlanta, Ga. Adequate adherence was defined as sufficient medication supply to ensure antifracture efficacy (MPR, 2-80%). The effects of patients' age, method of prescription payment, and pattern of past osteoporosis medication use on medication adherence also were evaluated. RESULTS: Of 211,319 study patients, 177,552 (84%) were taking weekly bisphosphonates vs 33,767 (16%) taking the daily prescription. Although significantly more patients taking the weekly compared with the daily bisphosphonates had adequate medication adherence, only about one third of patients in the daily dosing group and fewer than one half in the weekly dosing group achieved adequate adherence. Patients new to bisphosphonates had the worst medication adherence over the year (25.2% for weekly vs 13.2% for daily dosing; P < .001). The highest proportion of adequately adherent patients was among those continuing to take their existing weekly bisphosphonates; however, even in this group, only about 48% exhibited adequate medication adherence. CONCLUSIONS: Once-weekly dosing of bisphosphonates for the treatment or prevention of osteoporosis was associated with better medication adherence than daily dosing during a 1-year observation period. However, even with weekly dosing, adherence remained inadequate in more than one half of patients. These findings indicate that ways to improve medication adherence in the treatment or prevention of osteoporosis are still needed.