The UK medicines watchdog is to review the safety of a clot-busting drug often used to treat strokes.
The Medicines and Healthcare products Regulatory Agency believes the benefits of alteplase outweigh the risks.
But it is revisiting the evidence. Some experts say previous assessments may have been flawed.
The Academy of Medical Royal Colleges welcomed the investigation, saying the review has "huge implications" for patient safety.
A stroke is a medical emergency - and there is one every five minutes in the UK.
Most strokes are caused by a clot blocking the flow of blood to the brain.
Many patients are given alteplase to break down and disperse the clot - treatment known as thrombolysis.
Concerns over 'balance'
There is an increased risk of dangerous bleeding in the brain, but regulators have concluded this is outweighed by the benefits of improved recovery. However, some experts disagree.
Dr Roger Shinton, a stroke specialist, set out his concerns in a letter published in the Lancet.
He told the BBC: "My concerns using alteplase for stroke have always been that the risks of this drug are quite considerable, particularly with bleeding into the brain. The question is, do the benefits justify that risk? And I am not myself convinced that they do."
Dr Shinton said many stroke specialists shared his concerns.
Responding in the Lancet, the Medicines and Healthcare products Regulatory Agency (MHRA) said it had recently examined the issue and concluded that the balance of benefits and risks was still favourable. But it also confirmed it was setting up an expert working group "to ensure all relevant sources of evidence have been taken into consideration".
It said it expected the inquiry would report its findings early next year.
Jon Barrick, chief executive of the Stroke Association, said treatment by thrombolysis using alteplase was part of the great progress in stroke treatment.
"No treatment is without risks. The NICE [National Institute for Health and Care Excellence] recommendation on the use of alteplase is based on current evidence that shows overall the benefits outweigh the risks.
"Eligible patients treated with thrombolysis are more likely to have a good outcome and more people are surviving stroke and leading independent lives."
The manufacturers, Boehringer Ingelheim, say they stand by the safety of the drug.
"Our medicine is an emergency clot-busting treatment for someone who has just suffered a stroke. Our medicine increases the proportion of patients who are alive and leading independent lives following this life-changing event."
Dr Aseem Malhotra, consultant clinical associate to the Academy of Medical Royal Colleges, welcomed the MHRA announcement.
"We support the MHRA in carrying out an investigation as a matter of urgency as this has huge implications not just for patient safety but also for the provision of acute stroke services."