Milk Thistle or Silymarin

By Rania Gollakner, BS, DVM, MPH; Cheryl Yuill, DVM, MSc, CVH

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 preventing certain cancers, preventing chemotherapy side effects, and in protecting the kidneys.

Nutritional supplements are substances that can be used to supplement the diet, such as vitamins, minerals, amino acids, herbs, botanicals, enzymes, and probiotics. While many supplements are sold over the counter, they still contain ingredients that have biological effects and should be managed by your veterinarian. Follow your veterinarian’s directions and cautions very carefully, as they may be significantly different from those on the label.

There are differences in how countries regulate supplements. In the United States, these substances are not as vigorously regulated by the FDA as other medications, which means they can be sold without the manufacturer proving their efficacy and safety, and without a guarantee of consistent or accurately reported ingredients. In Canada, products that have been evaluated for quality, safety, and efficacy by Health Canada and are authorized for sale will have a license number on the label.

How effective is silymarin?

Limited studies in animals have been performed but there is anecdotal evidence that silymarin works to treat liver disease by inhibiting cell-damaging enzymes and by providing antioxidant and anti-inflammatory effects.

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; however, if vomiting occurs when dosed on an empty stomach, give future doses with food.

This medication can take up to a few weeks before full effects are noted but gradual improvements are usually appreciable. At times, effects may not be noted outwardly; therefore, laboratory tests may need to be done to evaluate this medication’s efficacy.

What if I miss giving my pet a dose?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time and return to the regular dosing schedule.

Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

Because studies are limited for this supplement, information regarding side effects is also limited. At this time, it appears to be well-tolerated in animals. In humans, upset stomach and diarrhea may occur, but the significance in animals is unknown.

This short-acting medication 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?

Because studies are limited for this supplement, information regarding risk factors is also limited. At this time, there have been no reported contraindications. Do not give it to pets that are allergic to it. Use cautiously in pregnant or nursing animals, as its safety in these conditions has not been established. Silymarin may affect estrogen, so use cautiously in animals with issues related to estrogen.

Are there any drug interactions I should be aware of?

Metronidazole (Flagyl®), antiviral medications, and medications that undergo liver metabolism should be used with caution when given with silymarin. 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.

Is there any monitoring that needs to be done with this supplement?

There is no specific monitoring that needs to be done while your pet is taking this supplement. Your veterinarian may monitor your pet to be sure that the medication is working.

How do I store silymarin?

Store this supplement at room temperature and protect it from light and humidity. There are many manufacturers of this supplement, so always follow the storage instructions on the package label.

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.

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