Does Milk Thistle Potentiate The Effects Of Coumadin?

Milk thistle has been used for more than 2,000 years to prevent and treat liver disease. Today, the herb is used to counter the toxic effects of pharmaceutical drugs and occupational exposure to chemicals. It is also the standard treatment for death cap mushroom poisoning. Like many medications, milk thistle can produce side effects and interact with other drugs. For instance, milk thistle interferes with the effects of blood-thinning drugs, including warfarin, also known as Coumadin. Rather than potentiate the effects of Coumadin, however, milk thistle decreases the drug's effectiveness. Consult your physician before using this herb if you take Coumadin or other medications.

Botanical Background

Milk thistle, or Silybum marianum, is a member of the daisy family of flowering plants and is native to the Middle East, Africa, Europe and the Mediterranean. Its long stalks support globe-shaped, purple flower heads surrounded by thorns, or thistles. The milk reference stems from the legend that a drop of mother's milk spilled upon the plant while the Virgin Mary was nursing her infant son, turning the veins of the leaf white. For this reason, milk thistle is regarded as a symbol of Mary in old wood cuttings and explains why the plant is called Marian thistle or Mary thistle in some parts of Europe.

Chemical Composition

According to the "Physicians' Desk Reference for Herbal Medicines," milk thistle leaf contains several flavonoids, including apigenin, luteolin and kaempferol glycosides. Beta-sitosterol is also present in the herb, in addition to fumaric acid. The seeds, from which traditional milk thistle extract is obtained, contain a flavonolignan mixture collectively referred to as silymarin.

Physical Effects

The silymarin compounds in milk thistle, most notably silychristin and silydiamin, exert hepatoprotective effects. Specifically, these agents protect against liver damage by blocking the ability of toxins to infiltrate cells and their capacity to attach to receptor sites. Although the University of Maryland Medical Center cautions that studies on the effects of this herb in humans are inconclusive to date, there is evidence that milk thistle may improve liver function in people with viral hepatitis, alcoholic cirrhosis or who exhibit liver damage from long-term use of certain medications, such as acetaminophen. However, the actions milk thistle compounds exert in the liver also affect how certain pharmaceutical medications are metabolized, such as Coumadin.

Interaction with Coumadin

The University of Maryland Medical Center warns that milk thistle may interfere with medications that are broken down in the liver by the same enzymes that metabolize silymarin and its derivatives. Based on previous clinical evidence that silymarin interacts with liver enzymes regulated by a gene labeled CYP2C9, scientists at the University of North Carolina tested the inhibition properties of isosilybin A and isosilybin B against the "S" form of warfarin in human liver microsomes. In the March 2010 issue of the "Journal of Pharmacology and Experimental Therapeutics," the study team reported that silybin B was particularly aggressive toward CYP2C9, but that both compounds inhibited warfarin. This clearly suggests that you should not take Coumadin or other anticoagulant medications in conjunction with milk thistle preparations.