Two drugs commonly prescribed to treat type 2 diabetes double the risk of heart failure, a study of data on more than 78,000 patients suggests.
The drugs may cause fluid retention, leading to heart problems
More than 1.5 million prescriptions for rosiglitazone and pioglitazone were issued in England alone last year.
The researchers, led by the University of East Anglia, suggest fluid retention caused by the drugs may be to blame.
Writing in the journal Diabetes Care, they call for a rethink on the drugs by the regulatory authorities.
Patients are advised not to stop taking the drugs, but to consult their GP if they have concerns.
Prescriptions for the drugs, known as thiazolidinediones, have doubled over the last three years.
Rosiglitazone, brand name Avandia, was linked to heart attacks by research published earlier this year.
The latest analysis of data estimates that one in every 50 people taking the drugs over a 26-month period will require hospital admission because of heart failure.
Lead researcher Dr Yoon Loke said: "This means that the diabetes drugs could have caused thousands of additional cases of heart failure, creating a substantial burden on hard-pressed NHS services."
The researchers also looked in detail at more than 200 cases of patients with heart failure related to the diabetes drugs and found that the problem developed even in patients taking low doses of the drugs.
While heart failure is often thought to be a problem affecting older patients, the researchers also found that one quarter of cases occurred in people younger than 60.
The manufacturers' information leaflets say that rosiglitazone and pioglitazone, brand name Actos, should not be used in patients known to have heart failure, but this research indicates that the drugs can provoke the problem even in those without a history of heart disease.
Occurs when the heart muscle is too weak to pump as efficiently as it should
Typical symptoms include breathlessness, swollen ankles and feet, and extreme tiredness
People with heart failure are more prone to potentially dangerous conditions such as: abnormal heart rhythms, stroke and heart attack
Dr Loke said: "Most patients in the studies did not have heart failure prior to starting on treatment with these drugs.
"There doesn't seem to be a group of patients who are safe from these side-effects."
Matt Hunt, of the charity Diabetes UK, said the drugs were recommended by the Medicines and Healthcare products Regulatory Agency (MHRA) as a safe and effective treatment for type 2 diabetes.
He said: "Obviously new evidence should be considered by the MHRA on an ongoing basis.
"Both the MHRA and the European Medicines Agency say that people should not stop taking glitazones and if concerned they should consult their doctor."
Both the EMEA and the MHRA said their experts were currently performing a re-evaluation of the benefits and risks of both rosiglitazone and pioglitazone in view of the concerns on cardiovascular safety.
This re-evaluation is intended to be finished later this year.
A spokesman from the drugs' guidance watchdog NICE said: "If the EMEA decides on the basis of this new research that the risks posed by these two glitazones outweigh their benefits, they will issue updated prescribing advice to healthcare professionals.
"This will take precedence over the existing NICE guidance."
Drug company GlaxoSmithKline, which makes Avandia (rosiglitazone), said: "The risk of heart failure in diabetes patients and with use of these medicines is well recognised and is clearly identified in prescribing information to doctors in the UK."