There are differences in the risk profile of patients prescribed alendronate, risedronate, or ibandronate. Observed reductions in fracture incidence over time suggest that the effectiveness of each bisphosphonate in clinical practice has been consistent with their efficacies demonstrated in randomized controlled trials.

Observational studies of bisphosphonate effectiveness for fracture prevention are subject to bias from unknown characteristics of baseline fracture risk at the start of therapy. The fracture incidence during the short period after starting a bisphosphonate and before any expected clinical benefit likely reflects baseline fracture risk. Bisphosphonate effectiveness may then be estimated by measuring the change in fracture incidence over time on therapy.

Administrative billing data were used to follow three cohorts of women aged 65 and older (total n = 210,144) after starting therapy either on alendronate, risedronate, or ibandronate in the USA between market introduction and 2006. Within each cohort, the baseline incidence of clinical fractures at the hip, vertebral, and nonvertebral sites was defined by the initial 3-month period after starting therapy. Relative to these baselines, we then compared the fracture incidence during the subsequent 12 months on therapy.

At the start of therapy, the ibandronate cohort was younger and had fewer prior fractures than either the risedronate or alendronate cohorts. Accordingly, the baseline incidence of hip fractures was higher in the risedronate cohort (0.90 per 100 person-years) and in the alendronate cohort (0.77) than in the ibandronate cohort (0.64). Relative to the baseline incidence, fracture incidence was significantly lower in the subsequent 12 months in both cohorts of alendronate (18% lower at hip, 28% at nonvertebral sites, and 57% at vertebral sites) and risedronate (27% lower at hip, 21% at nonvertebral sites, and 54% at vertebral sites). In the ibandronate cohort, the fracture incidence was lower (31%) only at vertebral sites.

Differences in the baseline fracture incidence among the cohorts may reflect differences in the risk profile of patients prescribed each bisphosphonate. The reductions observed in fracture incidence over time within each cohort suggest that the effectiveness of each bisphosphonate in clinical practice has been consistent with their efficacies demonstrated in randomized controlled trials.