Many drugs interact with coumadin and may cause more anticoagulation effect (clofibrate, diazoxide, ethacrynic acid, nalidixic acid, phenylbutazone, salicylates, aspirin, sulfonamides, alcohol, allopurinol, amiodarone, cimetidind, phenytoin, erythromycin, gemfibrozil, propranolol, thyroid drugs) or decreased anticoagulation effect (smoking, estrogens, vitamin K, aluminum hydroxide - antacids, cholestipol, spironolactone). The effects of coumadin must be carefully monitored by a blood test called an INR. Usually this is checked more often at the onset of taking the drug and less often once a steady state has been reached. Therapeutic INR is usually 2 to 3 depending on the condition being treated.

There are naturally occurring substances and foods that reduce platelet aggregation and act as natural blood thinners. In some circumstances the use of a prescription anticoagulant may not be necessary when using these. However, when on a prescription anticaoagulant, caution should be maintained regarding the use of large amounts of substances like green tea, ginkgo, garlic and vitamin E.