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Background: Vertebral fractures increase the risk of new vertebral fractures; however, we are not aware of any study addressing the risk of new vertebral fractures adjacent to existing vertebral fractures. Therefore, we sought to determine the influence of the number and severity of prevalent (preexisting) vertebral fractures on the risk of new adjacent vertebral fractures and to determine whether teriparatide (rhPTH [recombinant human parathyroid hormone] [1-34]) or raloxifene treatment reduces the incidence of adjacent vertebral fractures in postmenopausal women with osteoporosis.Methods: Data from the Fracture Prevention Trial and the Multiple Outcomes of Raloxifene Evaluation trial were analyzed to determine the incidences of new adjacent and new nonadjacent vertebral fractures in the placebo groups and the effect of treatment with raloxifene and teriparatide on the incidence of new adjacent vertebral fractures as compared with that of new nonadjacent vertebral fractures.Results: Of 1226 untreated postmenopausal women with one or more prevalent vertebral fractures at baseline, 196 (16.0%) had a total of 292 new vertebral fractures during the two-year follow-up period, with 108 (8.8%) of the 1226 women having at least one new fracture adjacent to a prevalent fracture. Of the 292 new vertebral fractures, 136 (47%) were adjacent to a previously existing vertebral fracture. The risk of a new adjacent vertebral fracture was 2.5-fold higher than the risk of a new nonadjacent vertebral fracture (4.03% compared with 1.59%). The incidence of new adjacent vertebral fractures increased with both the number and the severity of prevalent vertebral fractures. Teriparatide reduced the risk of any new, new adjacent, and new nonadjacent vertebral fractures by 72%, 75%, and 70%, respectively, compared with the rates in the placebo group. Similarly, compared with the placebo, raloxifene treatment reduced the risk of any new vertebral fracture, new adjacent vertebral fracture, and new nonadjacent vertebral fracture by 54%, 54%, and 53%, respectively.Conclusions: In untreated postmenopausal women with osteoporosis, nearly half of the incident vertebral fractures occur adjacent to an existing vertebral fracture. Both teriparatide and raloxifene can significantly reduce the occurrence of new adjacent and nonadjacent vertebral fractures. [ABSTRACT FROM AUTHOR]