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Silymarin (Milk Thistle)
Revised: January 07, 2020
Published: December 06, 2004

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Numerous herbal brands are available (usually as seed extract).

Veterinary brands include: Marin (which combines silymarin with vitamin E and phosphatidyl choline) and Denamarin (which combines silymarin with SAMe)


Milk thistle is a flower, specifically a member of the aster family. Its seeds and roots have been used for an assortment of medical purposes for thousands of years. Three biochemicals of interest have been isolated from the milk thistle: silychristine, silydianin, and silybin, which is also called silybinin and is considered the most powerful of them all. The mixture of these three substances is called silymarin. Silymarin has been traditionally used to treat liver disease and while it has recently been advocated for use in pets, most scientific information available concerns human use. The biological mechanism of action is yet unknown but several theories exist:

  • Silymarin may control cell membrane permeability, which means that silymarin may control what substances enter the interior of a cell.
  • Silymarin may inhibit inflammatory biochemical pathways.
  • Silymarin may have free radical scavenging properties, which means that it may absorb harmful reactive atoms that could damage other molecules.
  • Silymarin may increase protein production by liver cells.
  • Silymarin may stabilize mast cells (cells containing inflammatory granules.
  • Silymarin in high doses increases the flow of bile.

How This Medication Is Used

In human patients with cirrhotic livers, alcohol-induced hepatitis, and liver disease from Hepatitis C, silymarin has been shown to improve symptoms, laboratory tests, and survival. Silymarin has not been investigated formally for all liver diseases but it is a safe substance and should not cause problems if used in a liver situation where its benefit is unproven. In dogs and cats, silymarin has been shown to be helpful in cases of Aminita mushroom toxicity and it was protective after carbon tetrachloride administration (both are situations are toxic liver diseases).

Phosphatidyl choline is added to some oral preparations of silymarin as it enhances silymarin absorption into the body from the GI tract.

Side Effects

At doses greater than 1.5 grams per day the increased bile flow side effect may cause diarrhea. Side effects are rare but the following has been reported for humans: upset stomach, headache, joint pain, weakness.

Interactions With Other Drugs

Please keep in mind that herbal medications are not held to the same standards of efficacy and safety that other drugs are. Impurities in processing may include less innocuous plant biochemicals. Using a product from a reputable manufacturer is especially important.

Silymarin combined with cisapride, an intestinal motility enhancer, can lead to heart rhythm disturbances.

Concerns And Cautions

Silymarin is not recommended for humans during pregnancy. It is probably a good idea not to use milk thistle products in pregnant dogs until more information becomes available.

Milk thistle products should be stored at room temperature.

Because herbal medications are not held to the same purity and efficacy standards as other medications, there may tremendous variation in strength between brands or even between batches of the same brand.

We recommend checking with your veterinarian about what brand is felt to be reliable.

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