Heparin, a natural product isolated from animal liver, prevented thrombosis (blood clotting) during a blood transfusion in 1935, and became the most commonly-used anticoagulant (also called a blood thinner). It also prevents clot formation in cardiac and arterial surgery.
In 1955, wafarin (Coumadin), an orally-active anticoagulant that prevents strokes and treats heart attacks and thrombosis, was introduced as a new drug. During the 1970s, it was discovered that once clots had formed, they could be treated with thrombolytics. Utilizing enzyme activity to dissolve blood clots lead to Urokinase (1977), streptokinase (1978), and the genetically engineered recombinant tissue plasminogen activator, tPA (1987).