Warfarin sodium (Coumadin) is a commonly prescribed “blood thinner.” This medication interacts with no fewer than 460 other drugs; approximately 1/3 of these interactions are graded as “severe,” meaning the interaction could lead to serious medical consequences or even death.
Most people who take warfarin have been told to limit foods that contain vitamin K because this vitamin interferes with warfarin’s action. Unfortunately, these same individuals often take over-the-counter drugs and supplements—and are occasionally even prescribed new medications—that interact with warfarin as well.
An awareness of some of the more common drug-warfarin and herb-warfarin interactions is not just an esoteric exercise; this knowledge can prevent unnecessary medical complications as a result of impaired effectiveness of warfarin (excessive clotting) or augmented effectiveness (hemorrhage). At the very least, any substance that interferes with warfarin’s action will lead to more frequent—and more costly—blood tests to adjust the medication’s dosage.
Common Drug-Warfarin Interactions
Medications that may increase bleeding risks when taken with warfarin:
- Allopurinol, amiodarone, metronidazole (Flagyl), certain antifungals like miconazole (including intravaginal use), some cephalosporin antibiotics, aspirin, ibuprofen, naproxen, acetaminophen (Tylenol), clofibrate, heparin (including subcutaneous forms), Tagamet, fluoroquinolone antibiotics (Cipro, Noroxin, etc.), Antabuse, Prilosec, phenytoin (Dilantin), quinidine, propoxyphene (Darvon), statin agents (particularly lovastatin and pravastatin), sulfonamide antibiotics and other sulfur-based drugs, tamoxifen, isoniazid, zafirlukast, zileuton, alcohol (acute use), paroxetine (Paxil), thyroid hormones
Medications that may increase clotting tendencies when taken with warfarin:
- Oral contraceptives, phenytoin, penicillins, rifampin, chronic alcohol use induces liver enzymes that break down warfarin (cirrhosis, however, decreases warfarin breakdown and leads to bleeding), cholestyramine, sucralfate, aluminum hydroxide, colestipol, barbiturates, carbamazepine, glutethimide, griseofulvin, corticosteroids
Common Herb-Warfarin Interactions
There are relatively few documented cases of herbs interfering with the action of warfarin. However, some interactions (e.g., danshen, CoQ10, ginseng, ginkgo) have been reported in the medical literature (see below).
Documented complications of herb-drug interactions have included intracranial hemorrhage, bruising, hematuria, gastrointestinal and intra-thoracic bleeding, and changes in coagulation times. Concerns about other potential interactions are based on known pharmacologic properties of specific herbs.
Herbs or supplements that may increase bleeding risks when taken with warfarin:
- Vitamin E, soy-based products, ginkgo biloba, white willow bark, meadowsweet, feverfew, cranberry, fish oil and omega-3 supplements, bromelain, danshen, devil’s claw, dong quai, papain, garlic
Herbs or supplements that may increase clotting tendencies when taken with warfarin:
- Vitamin K, ginseng (although ginseng alone actually increases bleeding), coenzyme Q10, green tea (in large amounts)
(Adapted from Am J Health-Syst Pharm 2000;57(13):1221-1230; US Pharmacist 2000;25(8):42-53; MayoClinic.com January 2008)
Warfarin is a useful medication that has found wide use for its anticoagulation properties. It is frequently taken in concert with other prescription drugs and over-the-counter remedies.
Warfarin exhibits a fairly narrow therapeutic window (i.e., its therapeutic effect is only exerted within a narrow range of plasma concentrations for any given individual, and minor variations from this range can lead to problems). Therefore, other drugs, foods, or supplements that either reduce or augment warfarin’s effects can leave patients at risk for medical complications.
Individuals who take warfarin (Coumadin) should consult their health care professional before making significant changes in diet or adding new medications or supplements to their daily routine.
Stephen Allen Christensen - Dr. Steve Christensen's writing has appeared in magazines, professional journals, poetry anthologies, and children's books since 1976.
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