Some women with osteoporosis can stop taking a bone-protecting drug after five years without increasing their risk of fractures, say scientists.
The drugs protect against bone density loss
The class of drugs, bisphosphonates, are given to strengthen the bones of women who have gone through the menopause and risk fractures.
Doctors have been unsure how long women need to be on these drugs, some advising they be taken indefinitely.
The study is published in the Journal of the American Medical Association.
When to stop
The need to establish the optimum duration of bisphosphonates for postmenopausal women, who have weaker bones because of hormone changes, was raised by the government's treatment watchdog, the National Institute for health and Clinical Excellence (NICE) last year.
It said: "Given the emergence of evidence from one bisphosphonate that the benefits of the drug may continue for several years beyond treatment cessation, the Institute recommends that research should be carried out to define both the optimal duration of treatment with individual bisphosphonates, and the most beneficial age at which to start treatment."
The latest work found women who discontinued a bisphosphonate called alendronate after five years or more had the same rate of non-spine fractures as women who continued the drug.
However, women at high risk of spinal fractures are better off staying on the drug, say the scientists.
Severe spine fractures associated with painful symptoms were fewer in those who continued the treatment, but the difference was small - 5% in those who discontinued the drug versus 2.5% among the women who carried on taking alendronate.
The findings also suggests that women who wish to take the drug for 10 years can do so safely.
Lead researcher Professor Dennis Black, from the University of California, said: "This has important implications as it has not been known whether treatment of osteoporosis should be continued indefinitely.
"Because women with osteoporosis, particularly older post-menopausal women, often need to take multiple drugs, this would be welcome news for this group."
The study involved 1,099 women who were studied for a 10-year period.
A spokeswoman for the National Osteoporosis Society said: "Spinal fractures can cause severe pain and disability and so those women at highest risk should probably continue treatment.
"Also, if you have been prescribed a drug you should go back and discuss it with your doctor before you stop the treatment."
Osteoporosis, a disease that weakens the bones, occurs in about five in every 100 people in the UK.
It is around four times more common in women than men, and most common in women who have been through the menopause.