Long-term use of certain diabetes drugs could double women's risk of breaking bone.
Thiazolidinediones, including rosiglitazone and pioglitzone, had already been linked to a raised risk of fractures, as well as heart problems.
UK and US researchers have quantified the risk, and showed that using the drugs for more than a year thins the bones significantly.
It found no increased fracture risk among men.
Two million prescriptions were written for rosigliatzone and pioglitazone in the UK alone last year.
The European Medicines Agency carried out a safety review of rosiglitazone and pioglitzone last year, and concluded their benefits outweighed their risks.
But the researchers argued the drugs had relatively modest therapeutic effects, and the regulators should think again.
Lead researcher Dr Yoon Loke, of the University of East Anglia, said: ''Women with type 2 diabetes are already at an increased risk of fractures - with a near doubling in the risk of hip fractures - so any additional risk from thiazolidinedione therapy could have a considerable impact on public health.''
Dr Loke said the underlying cause of the effect of thiazolidinediones was unclear, and required further research.
One suggestion is that the drugs may cause fractures by replacing bone marrow with fat cells.
However, he stressed women should not stop taking the drugs without first taking medical advice.
The latest study, also conducted by researchers at Wake Forest University in North Carolina, examined data from 10 previous trials, involving a total of 13,715 patients.
It found that year-long thiazolidinedione use among elderly, postmenopausal women with type 2 diabetes resulted in one extra fracture per 21 women.
Among younger women, aged around 56, the figure was one extra fracture per 55 women.
There is no clear evidence that other drugs used to treat type 2 diabetes, such as metformin and sulfonylurea, cause an increased risk of fractures.
Recent research into thiazolidinediones has focused on the drugs' adverse effects on the heart and cardiovascular system.
One study found that they doubled the risk of congestive heart failure, while another found rosiglitazone was associated both with increased heart attacks and a doubling of heart failure.
Dr Victoria King, of the charity Diabetes UK, said: ''We really do need further evidence through properly controlled trials before we can conclusively link thiazolidinediones to increased risk of various bone conditions in humans and determine which groups of people may be at greater risk.''
In a statement, the Medicines and Healthcare products Regulatory Authority (MHRA) said fears that thiazolidinediones raised the risk of fractures in women had been raised before, and healthcare professionals notified.
The information leaflet providing with the drug to patients already contains a warning about fracture risk.
GlaxoSmithKline, which markets rosiglitazone as Avandia, said the safety and effectiveness of the drug was backed by one of the largest clinical trial programmes ever undertaken for any medicine, with 52,000 patients studied.