Drugs used to strengthen bones can actually have the opposite effect, scientists have warned.
Taking biphosphonates for more than five years alters the composition of bone - making it more brittle.
Older women were found to be most at risk of taking the cheap pills, which include Fosamax, Boniva and Reclast.
The study of 50 women aged 65 to 93 found using the drugs long-term can make bones more mineralised and harder.
Such pills protect millions of osteoporosis patients from potentially fatal fractures if they are taken daily.
The findings have potential implications for the treatment of the condition which affects more than three million Britons and 44 million in the US.
Taking biphosphonates for more than five years alters the composition of bone - making it more brittle, a study found
Study author Professor Eve Donnelly from Cornell University in New York said: 'It’s kind of a double-edged sword.
'It’s extremely good to prevent bone loss but the drugs will also slow this natural process - which allows turnover.
'That’s one of the cautions I’d like to impart. What we have observed is really the result of long-term treatment.'
How long should you use them?
US health officials recommend that patients use bisphosphonates for no more than three to five years. The NHS says such prescriptions should be re-evaluated after five years of use.
But some of the volunteers in the journal Proceedings of the National Academy of Sciences had used the drugs for up to eight years.