Oral bisphosphonate use and colorectal cancer incidence in the Women's Health Initiative.

Bisphosphonates are widely prescribed to increase bone density in postmenopausal women with osteopenia or osteoporosis. Aminobisphosphonates have numerous anticancer properties and reduce bone metastases in cancer patients. Several studies, including the Women's Health Initiative (WHI), have found that use of oral bisphosphonates is associated with reduced risk of developing breast cancer, but less is known about associations with other common malignancies in women such as colorectal cancer (CRC). A few case-control and retrospective cohort studies have reported decreased risk of CRC among bisphosphonate users. In contrast, a prospective cohort study found no association. We evaluated the association between oral bisphosphonate use and CRC incidence in 156,826 postmenopausal women, ages 50 to 79 years, who participated in WHI clinical trials and observational study. A detailed health interview was conducted at baseline, and bisphosphonate use was ascertained from an inventory of regularly used medications at baseline and over follow-up. A total of 1931 women were diagnosed with incident invasive CRC during a median follow-up of 12 years. Alendronate was the most commonly used bisphosphonate, accounting for >90% of the total person-years of use. The association between oral bisphosphonate use and CRC risk did not reach statistical significance (hazard ratio [HR] from multivariable-adjusted models = 0.88; 95% confidence interval [CI] 0.72-1.07; p = 0.19). Furthermore, we did not observe greater risk reductions for women with longer duration of use. Uncontrolled confounding may explain why previous studies have observed an association.