Heparin is an anticoagulant that inhibits reactions that lead to the clotting of blood and the formation of fibrin clots. While heparin will not dissolve clots that have already formed, the drug will prevent these clots from becoming larger and causing more serious problems. This drug is useful in preventing blood clotting during open-heart surgery, bypass surgery, and dialysis. Side effects include abdominal pain, back pain, bloody urine, constipation, and tarry stools.