Garlic thins the blood. This gives promising hopes in case of heart attack, blood clots in blood vessels . Garlic even in moderate amounts thin the blood, reducing its capacity to form clots within the arteries . The effect of garlic came dramatically to the attention of dramatic community in the mid 1970s. Researchers in India studied the members of the Jain religious sect there. Some Jains abstain completely from onions and garlic , while others eat large amounts of them. A third group eat a moderate amount. Otherwise the three population  are very similar, making their situation ideal for a controlled study of garlic. The high garlic group consumed more than a pound of onion and at least 17 garlic cloves weekly. The blood of the members of that group  had less tendency to clot than the blood of the other groups, and the group that ate no garlic at all had the highest tendency to clot.
The clinical importance of this thinning was shown in a clinical trial in patients with intermittent claudication . In this condition , associated with atherosclerosis , pain in the buttocks or legs  is caused by the reduced blood flow due to narrowed arteries . Garlic significantly improved the maximum walking distance in patients with claudication. The effects started after about 5 weeks of administration and that they correspond to a simultaneous thinning of the blood at that time. It appears that thinner blood was better able to circulate through the narrowed arteries. Studies shown that  maximum effect is obtained when garlic is taken in the raw state.
It is fund that one constituent ajoene , has blood thinning capabilities similar to aspirn. However ajoene is not present in any significant amount in many garlic preparations, and researchers have suggested that allicin or a close chemical relative is responsible.