Ischemic stroke happens when blood flow to the brain is blocked by a blood clot. Thrombolytic therapy (tissue plasminogen activator: tPA) is a strong clot-busting medication, given intravenously, that dissolve the blood clot and reconstitute the blood flow and oxygen to the brain. Tissue plasminogen activator (tPA) is the only medication approved by the U.S. Food and Drug Administration (FDA) for acute treatment of ischemic stroke. Tissue plasminogen activator (tPA) is most effective when given within the first 4.5 hours from the stroke onset.