A combination of drugs routinely given to patients with high blood pressure may increase the risk of developing diabetes, according to experts.
Over 5m Britons are treated for high blood pressure
But they say the risk is small and patients taking thiazide diuretics with beta blockers should not be concerned.
Nevertheless, they have suggested that doctors should no longer prescribe this combination to patients recently diagnosed with high blood pressure.
Doctors will get new guidelines for treating the condition later this year.
The National Institute for Clinical Excellence is drawing up those guidelines.
As part of its review, it asked leading experts to review medical research into the drugs commonly used to treat high blood pressure.
The experts looked at seven major trials involving over 70,000 people. They found a link between patients taking this particular combination of drugs and diabetes.
Professor James Mason, who led the review, said the risk was "small but measurable".
More than 5m Britons receive medication for blood pressure, which if left untreated can increase the risk of heart disease and strokes. Many of these will be taking thiazide diuretics with beta blockers.
Professor Mason said some patients would be better off continuing to take this combination of drugs while others may be better off changing their treatment regime.
"This shouldn't be a cause for concern for people who have already had a stroke or a heart attack because the benefits of this treatment far outweigh the risks," he said.
"But for other patients the benefit is quite small and the risk of diabetes is of the same order. There are other drug combinations available."
Professor Mason said patients should discuss their treatment options with their GP. But he added: "There is a very small increased risk. Patients do not need to worry."
In a statement, NICE said it would pubish its guidelines on treating high blood pressure later this year.
"The National Institute for Clinical Excellence is currently consulting on the second draft clinical guideline on the management of hypertension in primary care," it said. "The final guidance is expected in June 2004."
About 1.4 million people have been diagnosed with diabetes. Untreated diabetes can increase the risk of heart disease, stroke, kidney damage, amputations and blindness.
People with diabetes are two to three times more likely to suffer a heart attack compared to people who do not have the condition. It is the biggest cause of blindness among adults of working age in the UK.