Manufacturer’s dog supplement description: “Organically grown and freshly ground, Herbsmith Milk Thistle is a natural alternative to support healthy liver function. It’s great for a dog with allergies, digestive issues, or to combat the negative side effects of the harsher alternatives. The most active ingredient in milk thistle is silymarin, which has great antioxidant properties.”[tabs][tab title =”Overview”]According to the manufacturer, Herbsmith Milk Thistle herbal supplement for dogs supports a healthy liver function, and is great for canines with allergies or digestive issues, or to combat the negative side effects of the harsher alternatives. The manufacturer does not mention what these harsher alternatives are. The sole product ingredient is ground Milk Thistle.
Milk Thistle extract has been used for centuries in both humans and pets to minimize the effects of toxins or hepatotoxic drugs, and to treat liver and gallbladder disorders. It has also been claimed to have beneficial effects for dogs with diabetes and cancer.[/tab] [tab title =”How It Works”]The active ingredient in Milk Thistle is Silymarin, which is an antioxidant known to counter the effects of a number of toxins. It also works to minimize damage when the liver is under stress from some diseases, such as in viral infection.[/tab] [tab title=”Uses”]The supplement is often given after discussion with a vet as supportive therapy if the animal is taking drugs that may damage the liver for other conditions.[/tab] [tab title =”Side Effects”]Milk thistle may cause diarrhea. More rarely, it may cause nausea, bloating, gas, and upset stomach. Animals with certain types of cancer and breeding bitches should not take this supplement, as the active ingredient can mimic some of the effects of estrogen.[/tab] [tab title =”Dosage”]The product is available in three tub-sizes (75g, 150g and 500g) but the composition of the product is the same in each tub. It comes as a powder which is added to the food.[/tab][/tabs]
Herbsmith Milk Thistle for Dogs
Extract of Milk Thistle has been shown to have a protective effect on dogs’ livers in some cases of poisoning, and to be supportive if given alongside veterinary prescribed drugs that may damage the liver.
However, its effects are unproven in canine cancers and acute and chronic liver disease, though the limited research available tends to indicate a broadly positive effect, including decreased cell injury markers in disease, and decreased spread of cellular changes associated with fatty liver disease.
Milk Thistle Extract is not well absorbed from the intestine into the liver, and so it is questionable how much of the supplement actually reaches the intended point of action, especially as the recommended dosage regime is so vague.
Customer reviews of this product, primarily sourced from online places like Amazon.com, seem to be largely positive (except for palatability where the reviews are neutral) and anecdotally show beneficial effects, such as raised liver enzymes returning to normal parameters. Many users appear to have been recommended the product by a vet as supportive therapy while taking a hepatotoxic drug for another condition (e.g. NSAIDs for dogs with arthritis).
Milk Thistle, also known regionally as St Mary’s Thistle or Scotch Thistle, is the common name for the plant Silybum Marianum. Extract of Milk Thistle has been used for more than 2,000 years in treatment of liver diseases  and it is currently the most well-researched plant in the treatment of liver disease , with over 12,000 related scientific publications on the subject from 2002 to 2012 .
How Herbsmith Milk Thistle is prepared
In the Herbsmith’s preparation of the dog supplement, it is unclear from the product’s technical specification whether the powder is the complete plant which has been ground, or it’s made from the seeds only. Generally, Milk Thistle extract is produced from ground up seeds, as this is where the majority of the active ingredient Silymarin is located . According to Herbsmith’s promotional video, the plant is grown specifically in the US for this supplement (though the plant’s wild distribution is almost worldwide).
It is also unclear from the provided details whether the concentration of active ingredient is consistent from batch to batch. A competitor product’s website states that, “most standardized milk thistle products sold in the United States are standardized to reflect 80% silymarin. This means that silymarin, the active ingredient, makes up 80% of the total weight of the product, and the inactive ingredients make up the remaining 20%.”
There is no indication on Herbsmith’s own specification for their canine supplement as to whether their product is standardized in this way, but given the scale upon which these supplements are sold, this would be a reasonable assumption to make. However, if the product is standardized to a different percentage than 80%, then this obviously has an implication for carrying out a cost comparison and an effectiveness review of the Herbsmith product alongside competing Milk Thistle products for dogs.
Active product in Herbsmith Milk Thistle
The active ingredient in Milk Thistle is Silymarin. It’s a substance composed of a number of chemically similar compounds of which the major one is Silybin (50-70% of Silymarin). Silybin is also sometimes called Silibinin (same chemical with a slightly different naming convention), and is itself made up of two compounds called Silybin A and Silybin B. To confuse matters further, some chemists define Silymarin only as the standardized extract of Milk Thistle fruits and seeds  (i.e. not the “wild” extract), but since almost all Milk Thistle supplements are standardized, this is of lesser importance.
Research carried out in dogs has shown that Milk Thistle extract is not absorbed well from the intestine as a stand-alone product, but can be improved by complexing the Silymarin extract with other compounds [3, 4].
Effects against poisoning
Silymarin is a powerful antioxidant, and has been shown to be protective in dogs against the effects of Amanita Phalloides (death cap fungus) when given after poisoning. It’s effective both through antioxidant protective effects and by competitively inhibiting the transfer of the Amanita poison.
In other species (primarily rats in clinical trials research), Silymarin has also been shown to mitigate the effects of paracetamol, doxorubicin (commonly used in chemotherapy) and iron poisoning, among other toxins .
Effects against acute and chronic liver damage
Despite the volume of work published on the effects of Silymarin in acute and chronic liver disease, there is no conclusive evidence of its beneficial effect in pets in mitigating liver failure. A 2007 Cochrane Collaboration meta-review  showed no benefit to taking Silymarin for alcoholic liver disease or hepatitis B or C infection in humans, and suggested that further review was needed. But in the same year, a new study showed positive effects in the prevention of liver cancers.
Using purified Silybin in animals has shown decreased oxidative damage in viral hepatitis, as well as less hepatic glutathione depletion (see our article on SAMe for a fuller discussion of the importance of glutathione in hepatic support and lower hydrogen peroxide production (both cellular responses to oxidative stress, mitigated by Silybin’s action as an antioxidant). Administration of Silybin also results in prevention of hepatic mitochondrial dysfunction, decreased hepatic enzyme activities, and improved liver histology in fatty liver disease . This suggests that administration of Milk Thistle extract (or specifically the active ingredient Silybin) is beneficial in some cases of hepatic stress.
- Kren V, Walterová D. Silybin and silymarin – new effects and applications. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub. 2005 Jun;149(1):29-41. [pdf]
- Abenavoli L, Capasso R, Milic N, Capasso F. Milk thistle in liver diseases: past, present, future. Phytother Res. 2010 Oct;24(10):1423-32. doi: 10.1002/ptr.3207
- Hackett ES, Twedt DC, Gustafson DL. Milk thistle and its derivative compounds: a review of opportunities for treatment of liver disease. J Vet Intern Med. 2013 Jan-Feb;27(1):10-6. doi: 10.1111/jvim.12002
- Filburn CR, Kettenacker R, Griffin DW. Bioavailability of a silybin-phosphatidylcholine complex in dogs. J Vet Pharmacol Ther. 2007 Apr;30(2):132-8. doi: 10.1111/j.1365-2885.2007.00834
- Rambaldi A, Jacobs BP, Gluud C. Milk thistle for alcoholic and/or hepatitis B or C virus liver diseases. Cochrane Database Syst Rev. 2007 Oct 17;(4):CD003620. doi: 10.1002/14651858.CD003620.pub3
Disclaimer: The purported effects of the supplements or ingredients discussed in this article may, or may not, have been subject to rigorous scientific investigation. This article is an introductory document which is not intended to be a scientific review either endorsing or refuting the claimed effects of the supplements or ingredients. The information is NOT to be used for diagnosis or treatment of your pet. You should always consult your own veterinarian for specific advice concerning the treatment of your pet. The supplement has not been tested or tried by the author of this content.