Warfarin thins the blood and prevents the development of clots by counteracting the effect of Vitamin K.  Pulmonary and critical care specialists commonly use warfarin to treat blood clots that form in the legs (deep vein thromboses) and blood clots that form in the lungs (pulmonary emboli).  In addition, physicians use warfarin to prevent strokes in patients who have irregular heart rhythms, to prevent blood clots after total joint replacements, and to prevent the formation of blood clots on artificial heart valves among other things.  Patients taking warfarin need to be aware of certain risks and precautions with this medication.

Warfarin thins the blood and prevents the formation and spread of blood clots, but patients taking it are more susceptible to bleeding as well.  Falls or other injuries can lead to large bruises or internal bleeding.  Similarly, any laceration will bleed more if a patient is taking warfarin.  Conditions such as prior ulcer disease, intracranial bleeds and diverticulosis predispose patients to bleeding in the intestines, bladder, stomach, or even in the brain.

Warfarin needs to be monitored closely.  Physicians use a blood test called the International Normalized Ratio (INR) to monitor the dosage.  In most cases, including cases of blood clots in the lungs or legs, physicians try to maintain an INR level between 2 and 3.  Any level less than 2 will not be therapeutic and creates a risk of new blood clots forming.  Conversely, any INR over 3 increases the risk of bleeding.

In rare cases, patients may have allergic reactions to warfarin which manifest as itching, redness of the skin, or localized swelling.  In patients with rare, genetic abnormalities, a condition called Coumadin-induced necrosis can occur.  This is a very dramatic destruction of skin, subcutaneous fat, and muscle tissue several days after starting warfarin therapy. 

Warfarin is metabolized by specific enzymes in the liver.  The same sites in the liver break down warfarin as well as other drugs, including seizure medications, antibiotics, cardiac medications, birth control pills and alcohol.  For this reason, if a patient on warfarin starts taking a new antibiotic or seizure medicine, there will be less space available in the liver to break down the warfarin.  Consequently, the level of warfarin in the blood will increase.

Alternatively, if the patient takes seizure medicines, heart medicines, or theophylline for a long period of time, the capacity of the liver to break down medicines will increase.  If seizure medications, heart medications, or theophylline are suddenly stopped, the liver will focus its full attention on the warfarin, removing it from the blood stream and decreasing the blood level of warfarin below the desired level.

Foods that contain vitamin K will negate the effect of warfarin.  Patients on warfarin do not need to alter their diet dramatically, but they need to be aware of which foods contain large amounts of vitamin K.  If a patient on warfarin suddenly ingests a large amount of vitamin K, it will counteract the warfarin and decrease the level of warfarin in the blood stream.

Leafy green vegetables contain large amounts of vitamin K, including kale, spinach, broccoli, asparagus, collard greens, cabbage and cauliflower.  Chicken liver and beef liver contain large amounts as well, as do some vegetable oils. Patients taking warfarin might want to read the labels of any vegetable oils they use for cooking.