Later this month the medicines regulator, the MHRA, is due to complete its review into the clot-busting drug Alteplase, the frontline treatment used in many cases of stroke. A number of experts in the UK, US and Canada have raised serious doubts about the drug's safety and effectiveness. They are concerned about potentially fatal harm to patients through an increased risk of bleeding in the brain and they question the credibility of scientific research on which Alteplase was licensed. Supporters and regulators say any risks are outweighed by the benefits of improved recovery. BBC Health Correspondent Adam Brimelow assesses the evidence and the dilemma posed for doctors and their patients.