Shares in UK drug giant GlaxoSmithKline fell for a second day after US research said its diabetes drug Avandia raised the risk of heart attacks.
Glaxo is "very confident" about the drug's safety
Research published in the New England Journal of Medicine suggests the drug, which was approved in 1999, may raise the risk of heart attacks by 43%.
US regulators said they were aware of a "potential safety issue" but Glaxo said it was "very confident" about the drug.
Its shares closed lower once again on Tuesday, following a 5% drop on Monday.
Glaxo shares closed 19 pence, or 1.37%, down at 1371p.
The Food and Drug Administration said it was not asking Glaxo to take any action in light of the findings about Avandia, which has been taken by six million people since 1999.
But it has urged patients with known heart problems to consult doctors about their treatments for type 2 diabetes - which affects up to 20 million Americans.
"FDA is carefully weighing several complex sources of data, some of which show conflicting results, related to the risk of heart attack and heart-related death in patients treated with Avandia," said Steven Galson, head of its Center for Drug Evaluation and Research.
"We will complete our analysis and make the results available as soon as possible."
Scientists analysed the results of 40 trials of Avandia - also known as rosiglitazone - involving 28,000 people and concluded that 65% of deaths among diabetic patients could be attributed to heart disease.
"Unfortunately, rosiglitazone appears to increase, rather than decrease, the most serious complication of diabetes," said Dr Steven Nissen, chairman of cardiovascular Medicine at the Cleveland Clinic, which led the study.
But the FDA said the research conflicted with that of other studies, while other experts said any risk to patients was small.
Glaxo said it stood behind Avandia, both in terms of its safety and effectiveness.
"We believe its significant benefits continue to outweigh any treatment risks," the company said in a statement.
Glaxo reached a settlement with the US authorities in 2004 over allegations that it withheld negative information about its antidepressant pill, Paxil.
As a result, it agreed to publish clinical trial results for some of its medicines on the internet.
One analyst said the concerns over Avandia could be a major problem for the firm.
"Unless this can be refuted, which I rather doubt, then this is going to seriously damage one of the cornerstones of Glaxo going forward," said Paul Diggle, at Nomura Code Securities.