Warfarin, which in the United States and Canada is most often known by its brand name Coumadin, is a life-saving drug that's hard to dose correctly. Too little, and you aren't protected from the blood clots that can cause deep vein thrombosis, pulmonary embolism, heart attack, and stroke. Too much, and you might notice that your skin seems to have sprung a leak, or you might be at greater risk for serious internal bleeding or a dangerous hemorrhage if you get a cut.
The dosing of this vital drug is particularly tricky at first. Doctors determine how clot-prone your blood is with a test known as prothrombin time (PT). This is a laboratory reading of how long it takes your blood to clot. Prothrombin time can be expressed in terms of prothrombin ratio (PR) and international normalized ratio (INR). Usually the lab sends the results in terms of INR and the doctor looks for a value of 2 to 3. If you are on warfarin, your doctor will check these numbers every day when you first start using the drug. That's because the first few days your levels of anti-clotting proteins (protein C and protein S) will fall faster than the levels of the pro-clotting factors you want to reduce (proteins called factor II, factor VII, factor IX, and factor X). For a short while, Coumadin actually makes you more prone to blood clots. But within a few days your doctor will find the dose that keeps your blood clotting factors high enough to protect you against injury but low enough to prevent dangerous blood clots in your heart or circulatory system, unless you accidentally consume the wrong natural blood thinners.
Over the Counter and Natural Blood Thinners You Need to Avoid When You Take Warfarin
If you don't want to start bleeding for no obvious reason at all, you need to avoid the over the counter blood thinners and herbal blood thinners that can raise your INR. Some of these problem substances you will be warned about. But sometimes your doctor doesn't give you a complete list.
- Vitamin E can cause serious bleeding in people on warfarin. Up to about 10 percent of people who take vitamin E supplements while on the anticoagulant develop serious bleeding problems. A Mediterranean diet emphasizing lots of olive oil increases vitamin E levels which increases the harmful interaction. 
- Glucosamine  and glucosamine with chondroitin , used for joint problems, increase the bleeding.
- Quinine water can increase clotting times and raise the risk of bleeding .
- Using a lot of ginger products has been reported to cause nosebleeds in a patient who was on warfarin .
- Garlic is a problem only for people on warfarin if they have a variation of a gene called VKORC1. However, cranberry juice and cranberry tablets cause bleeding problems in up to 30 percent of people who use warfarin. 
- Dihydroepiandrostenedione (DHEA) probably isn't a good idea for people on anticoagulant therapy. Anything that increases the body's production of natural steroids increases PT time. 
- Ginkgo doesn't have an effect on aspirin users , and doesn't have an effect on warfarin users at first , but can probably both reduce and prolong the effects of warfarin if ginkgo is used repeatedly . If you are taking any kind of anticoagulant, avoid ginkgo.
- Curcumin and turmeric change the rate at which the liver breaks down warfarin (and also Plavix) . When the study was just run for a week, there was no change in clotting factors, but there would probably be unexpected variations in long term use. When it comes to clotting factors, "unexpected" is never something you want.
Your doctor will no doubt tell you that Tylenol, aspirin, gout medicines, thyroid replacement therapy, certain antifungal drugs, many antibiotics, and some anesthetics can increase your clotting time. Always make sure your doctors and your pharmacist know you are taking warfarin. However, there are also a kind of "reverse" blood thinners list of foods and substances that decrease your INR, making you more prone to clot when you are on warfarin.
What Can Counteract Warfarin?
Your doctor is sure to tell you that you can't just eat a salad, or a serving of kale, or any other kind of leafy greens any time you like when you are taking Coumadin (warfarin). That's because the way warfarin works is by interfering with the body's use of vitamin K1 to make clotting factors. However, the most potent sources of vitamin K1 are (in order of K1 content) basil, thyme, parsley, coriander, kale, amaranth leaves, collards, oregano, dandelion greens, turnip greens, potato chips, Swiss chard, and green onions. We don't usually think of potato chips as high in any vitamin, but if especially if they are made from dehydrated potatoes, like Pringles, they are. Vitamin K2, which is associated with anti-atherosclerotic actions and is found in certain cheeses and butter and also appears in supplements with vitamin D, does not interfere with warfarin.
Korean or red ginseng can interfere with warfarin's anticoagulant properties, although it won't make a huge difference (usually about 0.15 INR) . And taking more than 500 mg of vitamin C per day can also interfere with warfarin's ability to stop clots . That doesn't mean you have to give up orange juice or fruit for the sake of maintaining a healthy INR, but don't take vitamin C for a cold.