Women with osteoporosis who take drugs to help avoid fractures still need to have their bone density monitored, a Canadian study suggests.

Researchers who studied more than 6,600 women taking osteoporosis drugs found that for nearly one in five, bone mineral density at the hip actually decreased after the women started taking the medication.

The hip "is an excellent site for monitoring bone mineral density because it predicts fractures, can be measured with great reliability making it easier to detect small changes, and is not affected by age-related problems like spinal arthritis," said lead study author Dr. William Leslie, a radiology researcher at the University of Manitoba.

The findings suggest it may be time for some physicians to rethink their reluctance to get women bone mineral density tests after they start medication, Leslie said.

Whether to send women for these tests - also called DEXA scans, for dual-energy x-ray absorptiometry - once they start therapy "has been controversial," he added by email. "It is relatively inexpensive but adds to the cost of care while there has been little scientific data to answer the question of whether a change in bone mineral density while receiving treatment tells us anything about that person's ongoing fracture risk."

During menopause and afterward, the body slows production of new bone tissue and women can face an increased risk of osteoporosis.

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For the current study, researchers followed women for an average of 9.2 years starting when they were typically around 64 years old.

Most of the women were prescribed osteoporosis medicines known as bisphosphonates, which work by slowing down how fast the body removes old bone, allowing time to regrow bone and make fractures less likely. This family of drugs includes Fosamax (alendronate sodium), Actonel (risedronate sodium) and Boniva (ibandronate sodium).

Overall, 910 women, or about 14 percent, experienced fractures during the study period, including 198 with hip fractures, researchers report in the Annals of Internal Medicine.

Bone mineral density tests of the hip showed that about 30 percent of the women experienced an increase in density after they started taking drugs and another 19 percent of these women had decreases.

Compared to women with similar hip bone mineral density before and after starting osteoporosis drugs, the fracture risk for women with a decrease in total hip bone mineral density was 2.9 percent higher after five years and 5.5 percent higher after 10 years.

In contrast, the risk of fractures was 1.3 percent lower at five years and 2.6 percent lower at 10 years for women whose bone mineral density increased during the study.

One limitation of the study is that women didn't all wait the same amount of time between their initial bone mineral density tests and follow-up scans, the authors note.

The researchers, therefore, can't say what the best retesting regimen would be.

Because changes in bone mineral density occur fairly slowly in most women, "there is some controversy as to what time point following an initial baseline bone mineral density test should a repeat test be performed," said Dr. Matthew Drake, a researcher in radiology at the Mayo Clinic College of Medicine in Rochester, Minnesota.

If the test is repeated too quickly, it may be hard to tell if any measured change really represents bone loss, Drake, who wasn't involved in the study, said by email. Waiting at least two to three years before a repeat test may be better for many patients, he said.

"There are several potential reasons why bone mineral density could decrease after osteoporosis therapy is started," Drake added. "These include failing to continue taking the medication as directed; a lack of adequate calcium and vitamin D intake," or a decrease in weight bearing activities.