Warfarin is the most commonly prescribed anticoagulant, or blood thinner, in the United States. It is used to prevent clotting in individuals who have medical conditions, such as heart rhythm abnormalities or cancer, that increase their risk for stroke or for blood clots in their extremities. One of the problems associated with warfarin is its tendency to interact with other medications and herbal supplements. These interactions can either increase your risk for clotting or increase your chances of bleeding.
How Warfarin Works
In order to prevent excessive hemorrhage, your blood forms a clot whenever you sustain an injury to the walls of a blood vessel. Clotting is achieved through the aggregation of platelets, which are small cells that circulate through your bloodstream, and by the coordinated activity of several proteins called clotting factors. Some of these clotting factors must be modified by a special enzyme before they can become fully functional. Warfarin interferes with this enzyme, thus rendering these clotting factors less effective and decreasing your ability to form a clot.
Warfarin Metabolism
Like many medications, warfarin must be broken down by enzymes in your liver so it can eventually be eliminated from your body. Many of the enzymes that participate in this process belong to a class of molecules called cytochromes. The chief enzyme responsible for warfarin's metabolism is cytochrome P450 2C9, or CYP450 2C9. Once a molecule of warfarin is broken down by CYP450 2C9, it can no longer interfere with your blood's clotting mechanism.
Competition for Enzymes
Cytochrome enzymes can only process a single molecule of a given substance, like warfarin, at a time. In a sense, CYP450 2C9 acts like a metabolic security checkpoint. If a large number of molecules show up at the checkpoint, they must wait in line for processing. Since warfarin remains active until it is broken down by CYP450 2C9, a delay in processing essentially increases your blood levels of warfarin. Increased blood levels of warfarin, in turn, lead to a higher risk for bleeding.
Goji Berries and Warfarin
A 2005 "International Journal of Cardiology" review showed that goji berries enhanced the activity of warfarin and led to increased bleeding times among patients who took both goji and warfarin. In 2008, Chinese physicians described what appeared to be warfarin overdoses among people who took the medication and who also consumed Lycium barbarum, or goji berries. A 2001 report in "The Annals of Pharmacotherapy" reviewed a case where a woman whose bleeding time had been stable on warfarin therapy suddenly developed a markedly increased bleeding time after she consumed goji berry tea for four days. The authors of this study discovered that goji competed with warfarin for the CYP450 C29 enzyme.
Precautions
If you are taking warfarin, you have probably been warned about eating certain foods, such as kale or spinach, and about taking other medications without checking with your doctor first. Many common herbal preparations, such as garlic, bilberry, ginkgo and chamomile, interact with warfarin, and goji should be added to this list. If you take warfarin, your physician should know about every medication or supplement you take, and you should contact your health care provider before you add anything new.
References
- "American Family Physician"; Warfarin therapy: evolving strategies in anticoagulation; Horton J, Bushwick B; 2000
- PubMed.gov: Comparative pharmacokinetics of vitamin K antagonists: warfarin, phenprocoumon, and acenocoumarol.
- PubMed.gov: Cardiovascular pharmacotherapy and herbal medicines: the risk of drug interaction.
- PubMed.gov: Warfarin overdose due to the possible effects of Lycium barbarum L.
- PubMed.gov: Possible interaction between warfarin and Lycium barbarum L.
- "JASA 2"; Herbal-drug interactions: an overview; Irawati L, Joseph R; 2007
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