Hi Oswald ,
Plavix, similar to Aspirin, is an antiplatelet drug and is effective in treating arterial clots (in cases of stroke, heart attack, peripheral arterial disease) . Plavix stops platelets aggregation or prevents them from sticking together. This tends to prevent clot formation in the arteries, including coronary arteries or the arteries on the surface of heart. Its action is same as that of Aspirin, and is always better than Coumadin for someone with major risks in the arteries (heart attack, stroke). Plavix does not influence the PT/INR.. Plavix' action is not influenced by vitamin K intake.
Coumadin, on the other hand is an antithrombin drug similar to Heparin. Coumadin is effective in treating venous clots [in cases of DVT ( deep vein thrombosis ) or pulmonary embolism]. Coumadin stops blood from congealing. This tends to prevent clot formation in veins and inside the chambers of heart. Coumadin is a risky drug because one has to do blood tests repeatedly to make sure of the concentration of the drug. It is almost always given if a person has a history of deep vein thrombosis or pulmonary embolism, heart arrhythmias, mechanical heart valves, or blood factors that promote too much clotting .
Since the mechanism of action and indications of both the drugs are different , the drugs cannot be compared in this regard and should also not be substituted for each other . Hope this helps you . Take care and regards !