Warfarin and Supplements Don't Mix

A recent study points out the danger of taking supplements along with the blood-thinning drug warfarin, prescribed to keep blood clots from forming or growing larger. It found that eight of the ten most popular supplements affected warfarin activity so much that taking them required changing the prescribed dose.

The study isn't an indictment of supplements. Warfarin requires a highly individualized dosage and has a very narrow useful range: too little and the blood won't be thinned properly, too much and severe, life threatening bleeding may result.

An increase in bleeding risk was specifically linked to the use of cranberry, garlic, ginkgo and saw palmetto supplements, the researchers said.

The following supplenments all affected warfarin's effectiveness so much so that they prompted a need for adjustments in the drug's prescribed dosage:

• Glucosamine/chondroitin
• Essential fatty acids
• Evening primrose oil
• Co-enzyme Q10
• Soy
• MelatoninGlucosamine/chondroitin
• Ginseng
• St. John's wort
• Multi-herb products

Almost 20% of all Americans currently take some type of herbal or non-herbal supplement.

The study looked at how the 20 most popular herbal and the 20 most popular non-herbal supplements interacted with warfarin. Supplements to be tested were chosen based on 2008 sales data. The researchers then tested how these supplements affected clotting tendency and bleeding. More than half of the 40 supplements tested had a noticeable effect on warfarin activity. Two-thirds of these raised the risk of bleeding.

An increase in bleeding risk was specifically linked to the use of cranberry, garlic, ginkgo and saw palmetto supplements, the researchers said.

Both diet and lifestyle can alter warfarin's activity, so it's not surprising that supplements also do so. Because warfarin is such a high-risk medication to begin with, anything that alters its effectiveness, including supplements, can lead to serious consequences.

The study also points out the general need to consider what supplements you're taking whenever you are prescribed a drug and to inform your doctor of the supplements you are taking.

Most supplements either contain drugs or substances that work like drugs, even though they're available without a prescription. In essence, taking a supplement is a lot like taking a prescription drug. And many drugs interact with each other, changing their effectiveness. While warfarin may represent an extreme case, whenever you're taking more than a single medication, it's important to consider how they interact with and affect each other. The same holds true when you take a drug and a supplement, including vitamin supplements, together.

The study results were presented at the Heart Rhythm Society's 2010 annual meeting in Denver on May 13.