Tissue Plasminogen Activator is a component of blood that plays a role in the break-up of clots. In its pharmaceutical preparation, it is used to break up existing clots that will not respond to heparin or warfarin. It is one of the first drugs used when a patient is suffering from a stroke caused by lack of blood flow. It can also be used to treat embolism and heart attack. However, it's usefulness decreases as time passes since the onset of symptoms and, after about 3–4 hours, it is contraindicated for these conditions.