Here's what we know about garlic, and how it affects blood clotting:
Most ingredients of garlic, particularly
alliin, have been demonstrated to
inhibit the production and/or release of chemical mediators, such as platelet-activating factor (PAF), adenosine, ADP, and thromboxanes. Some of the compounds (alliin/allicin (and its degradation products), ajoens, and several other sulfur-containing essential oils) also act as antioxidants and cause
reduction of mobilization of intracellular calcium. The mechanisms for the above effects have been suggested to include inhibition/blockade of
cyclooxygenase and fibrinogen receptors on platelet membranes by some garlic compounds.
... The findings of two other studies have also implicated that garlic augments the antiplatelet and anticoagulant activities of aspirin/NSAIDs and other blood-thinning medications, particularly warfarin in association with increased risk of bleeding.
In terms of raw vs. cooked garlic. Crushing garlic releases an enzyme, called
alliinase, that causes the formation of
allicin.
Allicin then breaks down to form the beneficial organosulfur compounds. However, the heat from cooking can inactivate
alliinase. Researchers have found that crushing garlic and letting it stand for 10 minutes before cooking allows time for alliinase to work before heat inactivates it. So, the next time you’re making something with garlic, crush it first and add it to the dish toward the end of the cooking time.