Drugs to Avoid When Taking Warfarin

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Warfarin, sold under the brand names of Coumadin and Jantoven, is a powerful drug used to thin the blood and reduce the chance of developing dangerous blood clots. If you take warfarin regularly, consult your doctor before taking any drugs or dietary supplements since many products can interact with warfarin's effectiveness or cause negative side effects.

General Information

As of 2011, warfarin is the most commonly prescribed oral blood thinning drug in the United States, according to the website American Family Physician. It can be used in patients who have suffered from deep-vein thrombosis or to prevent stroke, heart attack and pulmonary embolism. Warfarin treatment requires careful monitoring due to its tendency to interact with diet and other medications. It can be prescribed for short-term anticoagulant use or for long-term use. In addition to drugs and supplements, patients who take warfarin regularly may need to avoid foods that contain vitamin K, such as leafy green vegetables and certain vegetable oils.


Some antibiotics, such as penicillins, quinolones, metronidazole and cephalosporins can cause unwanted side effects in warfarin users or decrease warfarin's effectiveness. For example, according to American Family Physician, a fair amount of evidence suggests that penicillins can cause moderately severe symptoms in warfarin users due to their effects on International Normalized Ratio, or INR. Ideally, INR should be between 2 and 3. According to Emergency Medicine News, taking antibiotics while on warfarin significantly increases the chances of developing an INR above 7.0, which makes patients much more susceptible to intracranial hemorrhage.

Over-the-Counter Medicines

Consult your doctor before taking any over-the-counter medicines if you take warfarin. Common drugs that can cause side effects include aspirin, which acts as a blood thinner, acetaminophen, ibuprofen and naproxen. American Family Physician recommends that patients who take warfarin only use non-steroidal anti-inflammatory drugs such as ibuprofen with food or antacids to reduce the possibility of stomach damage. They should also avoid taking more than 2 grams of acetaminophen each day. Acetaminophen can decrease the absorption and metabolism of warfarin.


The use of alcohol while taking warfarin has been widely researched. If you take warfarin regularly, you should always drink in moderation and avoid binge drinking. Damage to the liver causes the body to become more sensitive to warfarin, which can cause excessive bleeding. Drinking excessive amounts of alcohol in the short-term can decrease the metabolism of warfarin and reduce its anticoagulant effects. According to Drugs.com, you should limit your alcohol consumption to one or two beverages per day to avoid negative side effects.