Long-term use of some of the most common drugs prescribed to tackle stomach acid problems may be weakening people's bones.
Hip fractures are more common among the elderly
Researchers found a significantly increased risk of hip fracture among UK patients taking 'proton pump inhibitors' for more than a year.
They said doctors should consider the risk when prescribing such drugs.
The University of Pennsylvania study findings appear in the Journal of the American Association.
Millions of people in the UK take drugs such as omeprazole, frequently on a continuous basis, and they are even available directly from pharmacies.
Some research has already suggested that taking the drugs could interfere with the body's ability to absorb calcium to keep bones strong.
The Jama study looked at a large group of records from the UK General Practice Research Database. All the people involved were aged over 50, and some had been taking proton pump inhibitors (PPI).
The researchers found that people who had been taking the drugs for more than a year - not an uncommon scenario - had a 44% greater risk of suffering a hip fracture.
Taking the drugs for even longer seemed to increase the risk yet further.
Fall can be fatal
Hip fracture is one of the most significant causes of severe disability in older people - up to one in five people who suffer a fracture following a fall die within 12 months.
The costs to the NHS of hip fractures are also huge, not only immediately afterwards but in the cost of emergency hip replacement operations.
The researchers said that while the link between increased risk and taking the medication appeared clear, it was possible that the type, general health and age of patients taking regular PPI medication might contribute to the difference in some way.
They wrote: "Physicians should be aware of this potential association when considering PPI therapy and should use the lowest effective dose for patients with appropriate indications."
They also suggested doctors should make sure that elderly patients took calcium supplements alongside the drugs to try to lower the risk.
Since 2004, omeprazole has been available directly from UK chemists without prescription, although at a lower than the normal prescribed dose, and, accompanied by guidelines which recommend it only for short periods.
Dr Denise Hansford, a senior lecturer in pharmacy at the Robert Gordon University in Aberdeen, said that research into how patients fared under this reclassification was 'overdue'.
She added that so far, her research had suggested relatively few patients were taking advantage of the new arrangements to obtain proton pump inhibitors.
'Long way short'
Ken McColl, a professor of gastroenterology at Glasgow University, said that the study did not prove that the drugs were the cause.
He pointed out that many people with arthritis and other joint problems - who were already more prone to hip fractures - would be taking large doses of painkillers known to cause acid indigestion.
He added that they would therefore be more likely to be taking the proton pump inhibitor drugs to deal with it.
Although the study authors had adjusted their results to take account of this, he said, there was still the possibility that the results could be skewed.
"While this raises some interesting issues." he said, "it is a long way short of proving that PPIs are causing this."