Medication adherence and fracture risk among patients on bisphosphonate therapy in a large United States health plan.

The association between bisphosphonate adherence in the first 12 months after therapy initiation and subsequent fracture risk was examined. Patients were identified from a large, commercially-insured population with integrated pharmacy and medical claims. Eligible patients were aged ≥45 years, were new to osteoporosis therapy (no osteoporosis medication claims in prior year) with first (index) bisphosphonate claim between 1/1/2005 and 4/30/2008, and had continuous insurance coverage for ≥12 months pre- and post-index. Patients with fracture claims ≤12-months post-index were excluded. Adherence was assessed using the medication possession ratio (MPR) over 12-months post-index (i.e., sum of days' supply dispensed divided by 365 days). Patients with a MPR>0.8 were considered adherent. The follow-up period to assess incident fracture began at month 13. The analysis included 33,558 new bisphosphonate users with mean age (SD) 59.5 (9.3) years; 94.0% were female. Median MPR at 12 months was 0.61 for alendronate and risedronate; 0.58 for ibandronate. Proportionally more nonfracture patients (39.3%) had a MPR>0.8 compared with fracture patients (34.9%, p<0.001). In multivariate modeling of bisphosphonate users' experience, those with a MPR>0.8 had a 14% lower risk of subsequent fracture than those with MPR<0.5, after controlling for demographics, insurance type, select comorbidities, and other potential confounders (p=0.0459). In a large, commercially-insured population, suboptimal adherence with bisphosphonate treatment was associated with increased fracture risk even after controlling for potential confounders.