People who take bisphosphonates for osteoporosis may be at risk for serious atrial fibrillation (AF), or irregular heartbeats, according to a new study. The research, presented at CHEST 2008, the 74th annual international scientific assembly of the American College of Chest Physicians (ACCP), shows that people taking alendronate or zoledronic acid, two common medications to prevent or slow the occurrence of osteoporosis, were significantly more likely to experience serious AF, including hospitalization or death, compared with placebo.