Actilyse is used to treat a number of conditions caused by blood clots forming within blood vessels, including: heart attacks caused by blood clots in the arteries of the heart (myocardial infarction) blood clots in the arteries of the lungs (pulmonary embolism) stroke caused by a blood clot in an artery of the brain (acute ischaemic stroke). Actilyse contains the active ingredient alteplase. It belongs to a group of medicines called thrombolytic agents. Actilyse works by dissolving clots in the blood vessels. These clots cause disease by interfering with normal blood flow.