I'm frequently asked if taking aspirin will help prevent Deep Vein Thrombosis (DVT) in situations such as long haul international flights where there is a known increased risk of DVT. Before answering this question it is helpful to understand some basic information about blood clots.
Blood clots arise due to thrombosis anywhere in the arterial or venous circulation. DVTs occur in the venous circulation. Heart attacks and strokes occur due to blood clots in the arterial circulation. Venous and arterial blood clots are very different.
The process of blood clotting is thought to have evolved at least 50 million years ago. Blood clots are composed of two components; a blood protein called Fibrin and particles in the blood called Platelets. Platelets aggregate together like a mesh and Fibrin acts like glue to hold it all together to form a blood clot.
Anything that might inhibit the development or activity of either Fibrin or Platelets will impair the formation of a blood clot. Modern medicine has come up with therapeutic methods that do this because a blood clot can be life threatening. Think Heart Attack, Stroke or Pulmonary Embolus.
Aspirin interferes with Platelet aggregation by irreversibly inhibiting Thromboxane A2, which makes it useful in preventing arterial blood clots which cause heart attacks and strokes. But Aspirin does not interfere with Platelet aggregation mediated by Thrombin which explains why it is not effective in preventing DVT which occurs in the venous system.
Heparin and Clexane do interfere with Thrombin generation and therefore are effective in preventing DVT.
If you are going on a long haul flight and want to minimize the risk of DVT then wear compression stockings, take a walk up and down the aisle for ten minutes every few hours. Also, make sure you drink plenty of fluids - that way you'll need to walk down the aisle to go and visit the loo!