Silymarin (Milk Thistle)

What is silymarin?

Silymarin, also called milk thistle, is a nutritional supplement used to treat a variety of liver conditions, diseases, and toxicities in cats, dogs, birds, rabbits, horses, and other companion animals. It may also have some efficacy in helping to prevent certain cancers, preventing chemotherapy side effects, and protecting the kidneys.

The U.S. Food and Drug Administration (FDA) does not review non-drug health products (e.g., herbals, nutraceuticals, shampoos, and other supplements) for safety or effectiveness before they are sold to the public. Manufacturers of non-drug health products must ensure their products are safe and labeled accurately. 

The FDA requires that these non-pharmaceuticals contain a disclaimer that says, “This product is not intended to diagnose, treat, cure or prevent any disease.” Health Canada licenses “veterinary health products” after reviewing safety, efficacy, and manufacturing information provided by the manufacturer. These products are not tested by Health Canada.

Your veterinarian is the best source of information about the safety of non-drug health products in pets. Just because a product is natural does not mean it is safe. All medicines and medicinal plants are potentially toxic if used inappropriately or given at high doses. Supplements can also have side effects, like pharmaceuticals. Few supplements and herbs have been subjected to rigorous scientific trials to determine their efficacy.

Although the information in this handout may be helpful, it also may be incomplete and may not represent the full range of effects the supplement could have on your pet.

How is silymarin given?

• Silymarin is given by mouth in the form of a capsule or tablet.
• It may be given with or without food. 
• If vomiting occurs when dosed on an empty stomach, give future doses with food.
• Follow your veterinarian’s instructions on administering this supplement carefully and give it for the entire time prescribed by your veterinarian, even if your pet is feeling better.

This supplement can take up to a few weeks before full effects are noted, but gradual improvements are usually noticeable. At times, effects may not be noted outwardly, so laboratory tests may be needed to evaluate its effectiveness.

What if I miss giving my pet a dose?

If you miss a dose, give it as soon as you remember. However, if it is almost time for the next scheduled dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once or give extra doses.

Are there any potential side effects?

This supplement appears to be well-tolerated in animals, although studies are limited, so information about side effects is also limited. In humans, upset stomach and diarrhea may occur, but the significance of these effects in animals is unknown. This short-acting supplement should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this supplement?

DO NOT USE silymarin in:

• Pets that are allergic to any component

USE WITH CAUTION in:

• Pets that are pregnant or nursing
• Pets with estrogen-related disorders

Are there any drug interactions I should be aware of?

Several medications should be used with caution when given with silymarin, including:

• Medications that undergo liver metabolism (e.g., metronidazole)

Vitamins, herbal therapies, and supplements have the potential to interact with each other, as well as with prescription and over-the-counter medications. It is important to tell your veterinarian about any medications (including all vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment.

Is any monitoring needed with this supplement?

• Your veterinarian will let you know when and how they want to reassess your pet to ensure the supplement is working and not causing side effects.
• At home, monitor for any abnormal changes. If any occur, discontinue the supplement and contact your veterinarian.

How do I store silymarin?

• Store this supplement at room temperature and protect it from light and humidity.
• Always follow the storage instructions on the package label.
• Keep the container out of reach of children and pets. 

What should I do in case of an emergency?

If you suspect an overdose or an adverse reaction to the supplement, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet's weight and symptoms. Report any adverse effects to the FDA or Health Canada.