Heparin is a commonly used blood thinner. It is usually injected intravenously into patients who are suffering from a clotting problem and is commonly used to treat symptoms of a stroke, heart attack or any other infarction that may be caused by clotting. It is also commonly used to treat deep-vein thrombosis, pulmonary embolysm, on bypass machines, It is also used on some medical equipment, such as the tubing on dialysis machines, to prevent blood from forming clots on their surfaces. It is derived from the mucous of slaughtered animals, generally pigs and cows. Heparin is found naturally in the body, but it's role in preventing blood from clotting in the body is poorly understood. It is currently believed that heparin in the body is not generally to prevent clots, but to fight off bacteria and other diseases at the sites of injuries.

Although heparin will not break up clots, it will prevent clots from growing or forming. This will generally allow the body's own anti-coagulating functions to take over to break up existing clots.

Heparin is destroyed by the digestive system, so it must be injected into veins or just under the skin. It cannot be injected into muscle without risk of forming hematomas. It is also processed quite quickly by the body and must be given continuously, usually in intravenous fluids.

The main risk in using Heparin is thrombocytopenia, an autoimmune condition where the immune system attacks the platelets that allow clotting. In addition, an overdose can result in massive internal bleeding, often leading to death.

Heparin is one of the oldest drugs still in continuous use, having been discovered in 1916 and put into clinical trials in 1935. It was discovered at Johns Hopkins University and further developed at the University of Toronto.

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