Since the blood stickiness test is so easy to do, it has led to probably the most interesting and sophisticated series of research projects which exists on garlic. Drs. Makheja, Vanderhoek , and Bailey , of George Washington University, Washington DC, took the anti-coagulant properties of garlic and onion seriously. They split garlic oil into various chemical groups, testing each one on blood stickiness. One of these was so effective that, while the normal blood stuck together completely in five minutes, the garlic-treated blood first stuck a little and then returned to a permanently liquid state.
Garlic’s sulphur components are again responsible. It seems that they interfere with the enzymes which make prostaglandins. These local messengers are found in platelets, as everywhere else in the body, and they control the way platelets clump. Garlic also reduces the stimulus to clot in the walls of the blood vessels themselves. It does so by restricting the catalysts which make thromboxane ( which promotes clotting ) and encouraging the opposite, prostacyclin, which delays clotting, Aspirin works in a somewhat similar way.
 You may feel that it would be unwise to thin the blood and delay the clotting process. In fact, in the modern world most people’s blood clots too easily. It goes with cholesterol and fats; if these are excessive, clotting is faster. Garlic brings it back to normal. Large quantities of garlic can be consumed without any danger of excessive bleeding. Nevertheless it would be wise to check with a doctor about taking garlic along with anti-coagulant drugs, and not to take garlic just before surgery.

Stephen Fulder & John Blackwood