Merck

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has won approval of a combined therapy for brittle bone disease.

The Food and Drug Administration approved the once-weekly Fosamax Plus D, an osteoporosis treatment containing the drug Fosamax and seven days' worth of Vitamin D. Launched in 1995, Fosamax reduces the risk of hip and spine fractures in postmenopausal women, the group most at risk for osteoporosis. Vitamin D promotes calcium absorption, which helps bone density and bone growth.

About 70% of women age 51-70 and 90% of women over 70 aren't getting enough vitamin D from food and supplements, according to a National Health and Nutrition Examination Survey published last year.

By 2020, half of women over 50 will have an osteoporosis-related fracture in their remaining lifetime, with the risk of fracture increasing with age, according to the Surgeon General.

The approval of Fosamax Plus D comes just days after the FDA approved once-a-month Boniva, which is co-marketed by Britain's

GlaxoSmithKline

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and Switzerland's Roche. GlaxoSmithKline shares fell 14 cents to $45.86, while Merck dropped 9 cents to $33.40.