Milk Thistle in Dogs and Cats

Milk thistle is an herbal product that is widely recommended and used by veterinarians. Like glucosamine, it is a supplement which has leapt over the gap between alternative and conventional medicine. Unfortunately, like glucosamine, this acceptance has come about on the basis of pretty weak evidence.

The active ingredient is a cluster of compounds called silymarin. There has been extensive in vitro research on silymarin, and it has a wide range of potentially useful effects. It appears to interfere with pro-inflammatory chemicals, functions as an anti-oxidant, and may interfere with the metabolism of some chemicals into toxic compounds in the liver. It also has some activity which could be potentially harmful, including interfering with the metabolism of a number of drugs and stimulating the effects of hormones like estrogen. As usual, these laboratory findings indicate the possibility of useful clinical effects, but most compounds that have these sorts of potentially useful effects in test tubes don’t work out to be good medicines.

The primary uses of silymarin in humans are to protect against or treat liver damage from toxins and infectious diseases, to improve the condition of diabetics, and to protect the kidneys from toxins. Some of these uses are based on traditional folklore, but as usual there are many traditional uses no longer recommended and for which there is not yet scientific support, including disease of the spleen, uterine disease, malaria treatment, appetite stimulation, stimulation of lactation, and others. In dogs and cats the primary use of for non-specific “support” of the liver regardless of the specific disease.

In humans, clinical trial evidence is mixed. A couple of studies have suggested it reduces insulin resistance in diabetic and may lower blood lipid levels. A Cochrane review of 13 studies including 915 people “could not demonstrate significant effects of milk thistle on mortality or complications of liver disease in patients with alcoholic and/or hepatitis B or C liver disease.” High quality trials were negative, and low quality trials suggested a benefit.

Very little research exists in dogs and cats. A small study of 20 cats given acetaminophen, a known liver toxin, found that those given a single oral dose of silymarin did not show the elevation of liver enzyme levels seen in those not given the compound. A similar study of dogs found some differences in elevations of kidney values between those that got silymarin and those that didn’t following exposure to a kidney toxin, though there was not a completely consistent pattern.

A study done in 1978 showed that dogs given a toxic mushroom compound orally and then given silymarin intravenously did not show the increase in liver values that was seen in control dogs. Another in 1984 found that 30% of the control dogs died whereas none of the dogs given IV silymarin along with the mushroom toxin died, and the livers from the treated dogs did not appear damaged by the toxin.

As far as risks, there appear to be few. Nausea, diarrhea, and other gastrointestinal effects are sometimes seen, and allergic reactions have been reported in humans.

So overall, the in vitro and laboratory animal evidence indicates it is plausible that milk thistle extract might have beneficial effects, though harmful effects in some situations could be expected as well. In humans, the clinical trials show weak evidence for benefit in diabetics and inconsistent but generally negative evidence for benefit in alcoholic or hepatitis-associated liver disease. Very little experimental, and apparently no high quality or controlled clinical research exists in dogs and cats. What there is suggests a benefit is possible. But we must bear in mind that preliminary, low-quality trials of milk thistle in humans looked promising but were not supported by subsequent better quality trials.

A clinical trial comparing animals with naturally occurring liver disease treated identically except for receiving either milk thistle or a placebo would be quite useful. In the meantime, use of the compound is not unreasonable given the suggestive low-level evidence, but it is not much more than a hopeful shot in the dark at this point.

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29 Responses to Milk Thistle in Dogs and Cats

  1. Bartimaeus says:

    My impression from looking into this topic myself is that Milk Thistle shows some promising preliminary results for certain types of acute toxin exposures-the mushroom toxicity you discussed-and negative results for things like chronic inflammation, neoplasia, etc. Unfortunately, many of the veterinary products are being marketed for use whenever liver enzymes are elevated, no matter what the cause. I am afraid that some veterinarians may be doing some basic bloodwork, finding elevated liver enzymes, and then prescribing a milk thistle supplement without identifying the cause of the elevated enzymes. Often it is hard to identify the cause of a mild transient elevation of ALP for example, but giving milk thistle without any attempt to follow up or rule out more serious problems seems like a bad idea. Of course, many of these cases will resolve without any treatment, which gives the proponents of milk thistle lots of anecdotes to promote their product. On the other hand, it would be interesting to know exactly which types of toxins milk thistle would be useful for treating-in humans an injectable form of silymarin has been used in some severe cases of mushroom poisoning with apparently good results.

  2. skeptvet says:

    Yes, Bartimaeus, that’s exactly the problem. There is a blind, almost automatic response on the part of many vets to say Increased Liver enzymes=Milk Thistle. This is not the kind of shotgun approach that’s likely to make good use of whatever medicinal potential the compound might have. Research is needed to identify what conditions, if any, it is appropriate for and in what dose/form in order to make the use of it rational and so effective.

  3. Art says:

    Are vets selling this stuff or just telling people to get it somewhere else?
    Art Malernee dvm

  4. skeptvet says:

    The major commercial veterinary product is Denamarin, from Nutramax. Some vets sell this, others sell human-labeled products, and probably some send people to GNC or wherever to get it.

  5. Art says:

    Let me guess. The two animal products are only sold by vets and cost more than the human products you can buy at GNC.
    Art Malernee dvm

  6. v.t. says:

    Art, Denamarin is a supplement, doesn’t require a prescription from a vet, and one can get it from many online pet stores or pet pharmacies (same applies to other brands). Whether the price is jacked up by the manufacturer, a vet, or 1-800-petmeds, does it matter? It’s a supplement, a multibillion dollar industry.

    Have you seen the prices for supplements and crap (yes, it’s crap) at GNC lately?

  7. Lori says:

    I am under the impression that Denamarin does require a prescription – it this incorrect?

  8. skeptvet says:

    As a nutritional supplement, regulated under the Dietary Supplement Health and Education Act, Denamarin does not require a prescription and cannot be sold with specific claims for preventing or treating disease, only general claims about “supporting” normal physiological functions and organ systems. However, I believe Nutramax has chosen to restrict the sale of this supplement to veterinarians only. There have been questions about the legality of such practices in the past (notably for flea control products, which used to be sold exclusively through veterinarians but are now available elsewhere as the result of litigation), but it is common for diets and supplements intended for use with specific medical conditions even when they do not quality for an official FDA label that would require a prescription.

  9. v.t. says:

    Most of Nutramax Lab’s products are available at numerous online pet stores, including Denamarin. A google search will reveal that.

  10. Denise Barker says:

    From what I am reading Denamarin seems to be directed at liver issues. Are there any results for canine renal failure?

  11. skeptvet says:

    There are a couple of in vitro papers looking at possible protective effects on kidney cells from rats, but it generally isn’t used for renal disease, and I’m not aware of any evidence supporting this use.

  12. Tennille says:

    I have a 15 year old Siberian Husky who’s had two cancer surgeries for spindle cell tumors and is being treated with prednisone for another tumor on her larynx which has improved, giving her back the ability to eat and breath with more ease. Her first surgery, the liver enzymes were over 800, the second surgery they were over 1300, and within the last year they’ve increased to over 1800. My vet has done a CBC, x-rays, ultrasound and considered all factors prior to suggesting milk thistle. The only thing I know for sure is that after 1 month of administering milk thistle, I asked my vet to take a blood sample, the results are impressive and came back at 306 which is a fairly substantial reduction, in my humble opinion. I would agree it shouldn’t be suggested all “willy nilly” and the source of the problem should be explored, but I also don’t think natural health alternatives should be so quickly dismissed as voodoo. You have to bear in mind very few pet parents are willing to pay copious amounts of money looking for the problem and a solution.

  13. skeptvet says:

    I don’t think you could find anything in my post suggesting milk thistle is “voodoo.” If you read it, you would see that I discuss the existing evidence suggesting it has uses in some conditions, and I have offered to my own patients with a clear disclosure of the limitations of current evidence. Your anecdote, of course, is compelling to you but doesn’t really help us in deciding when or if milk thistle should be used. And, of course, “natural” is a meaningless term in medicine. Be careful of assuming that a recognition of the limitations of anecdotes and the advantages of controlled research evidence is the same thing as an unthinking dismissal of a hypothesis.

  14. Richard says:

    I too had a Siberian that had elevated liver enzyme levels – no side effects but were picked up by a blood test prior to an operation to remove a eye, non cancerous growth. I went the milk thistle route and the blood test a month later saw the levels come down. Was it the milk thistle, was it a random infection that happened to disappear over that month, I have no idea and I guess I will never know. Have not returned to the vet for a follow up blood test , pups is 13 now and still doing ok apart form some arthritis. It was nice of the vet to suggest that and perhaps some dogs can have elevate liver levels and function just fine. I’m sure if he was really sick I’d been more included to hit him with some conventional drugs.

  15. skeptvet says:

    Often with mild to moderate liver enzyme elevations with no obvious cause, we choose to monitor monthly. They very often do go down by themselves, which is why controlled studies are so important in evaluating things like milk thistle. As you say, always happy for a good outcome but it doesn’t really help us know about the effects of this treatment.

  16. Mfernflower says:

    @Bartimaeus I know this was posted in 2011, But I am new to this site and loving it! From what I can remember Milk Thistle was only shown to work as an antidote to ?-amanitin, A toxic orally active cyclic peptide produced by the mushroom Amanita bisporigera (among a few other species) that causes liver damage by inhibiting RNAP II.

  17. Mfernflower says:

    The ? should be the Greek alpha symbol but wordpress does not seem to support it!

  18. SkeptMD says:

    I’m ashamed to admit I allowed a VCA vet to scam me into buying denamarin, following carprofen poisoning. I also allowed them to scam me into buying 3 gastric protectants (misoprostol, omeprazole and sucralfate) after the 48 hours of inpatient treatment. All labs normal, everything normal, why didn’t I question them? Maybe not worth my time because they were all so resistant to any inquiry as to the efficacy, risks and benefits of the treatments. All three stomach meds, how can that be necessary? We wouldn’t do it in humans, why in dogs?

  19. art malernee dvm says:

    VCA has been bought or is about to be. We now have a group of veterinary hospitals that make up about 10% of all the hospitals in the usa. the good news is if you look at the group I thing you will find none of them selling homeopathic medicine like the fancy vet hospital down the street from me and the privateMD on my insurance list I went to that was selling homeopathic medicine in his waiting room. That’s the good news. The bad news is so far I have not seen one of these 10% of the vet hospitals in the usa not vaccinating for something with the same vaccine yearly. Pet vaccines are not reformulated every year like human flu vaccines. Write the letter. I suspect it will go up the chain to corporate. Its like writing a letter to 10% of the vet hospitals in the usa. That’s got to be more effective than a letter that one vet reads.

  20. Wendy Tunnard says:

    My red and white Border Collie, Sparky was born diabetic and he us now 14 months old. He was hand reared from 4 days old as could not suckle due to throat problem and his mother abandoned him. In his short life he has had acute pancreates, liver failure and now kidney failure. He is currently non a drip a the vets. The UK diabetic dog group recommended milk thistle during his over failure episode when the enzyme result went up to 2834 and should be below 100. Vets could not get it below 1800 and on his release to home I was told by a vet at the Animal Health Trust that the level would never drop beneath 800 due to the severe damage to his liver. I used milk thistle an threw away all the meds he had been prescribed except or Denemarin and put the milk thistle powder in his two daily meals. Within 2 weeks his emzeyme levels dropped to 100. I was ecstatic! Vets were amazed but would not commit to believing milk thistle had done it. Now he has kidney failure I do not know if the milk thistle has contributed to it or the Denemnarin has or the Diabetes or insulin injections are the cause. Any ideas?

  21. skeptvet says:

    I’m sorry you are dealing with such a difficult set of problems. Unfortunately, I don’t think it is possible to find a simple answer to your questions. Milk thistle may or may not have been responsible for the improvement you saw. And while there is no reason to think it can cause kidney disease, there is a lot we don’t know.

  22. v.t. says:

    Wendy, if I may suggest – if your current vet isn’t a specialist, perhaps he could consult one on your behalf. That’s a lot for a young pup to be dealing with. A consult might give you more direction, in case something else has been missed, please give it consideration. Here’s hoping your pup can recover!

  23. Andrew says:

    Skeptvet – do you still hold the same view of milk thistle stated above in 2011?

  24. skeptvet says:

    I haven’t looked into the issue in a while. A quick database search doesn’t show much new evidence since 2011. The systematic review below is for humans, and is about as unimpressive as earlier reviews. I don’t see any new clinical trials in dogs and cats, so for now I’d have to say it still looks plausible but with only minimal evidence for meaningful, real-world benefits.

    World J Gastroenterol. 2017 Jul 21;23(27):5004-5017. doi: 10.3748/wjg.v23.i27.5004.
    Effect of silymarin on biochemical indicators in patients with liver disease: Systematic review with meta-analysis.

    de Avelar CR1, Pereira EM1, de Farias Costa PR1, de Jesus RP1, de Oliveira LPM1.
    Author information
    To evaluate the effect of silymarin on the serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST) and gamma glutamyl transpeptidase (?GT) in patients with liver diseases.
    A systematic review with meta-analysis of ramdomized and controlled clinical trials was performed, evaluating the effects of sylimarin in patients with hepatic diseases, published by January 31, 2016. Clinical trials were sought on the basis of The Cochrane Central Register of Controlled Trials in the Cochrane Library, PubMed/Medline, Scopus, Web of Science, Lilacs and Clinical Trials. The trials with adult and elderly patients of both sexes, with Liver Diseases who took oral silymarin supplementation, as extract or isolated, as well as Silymarin combined with other nutrients, were included. The trials should provide information about the intervention, such as dosages and detailing of the product used, besides the mean and standard deviation of serum levels of ALT, AST and ?GT of the baseline and at the end of the intervention.
    An amount of 10904 publications were identified. From those, only 17 were included in the systematic review and 6 in the meta-analysis, according to the used selection criteria. In this meta-analysis, the results indicated a reduction of 0.26 IU/mL (95%CI: -0.46-0.07, P = 0.007) at the level of ALT and 0.53 IU/mL (95%CI: -0.74-0.32, P = 0.000) at the serum levels of AST after using the silymarin, both, statistically significant, but with no clinical relevance. There was no significant change in the ?GT levels. Subgroup analyzes were also performed for the biochemical markers in relation to the type of intervention, whether silymarin isolated or associated with other nutrients and the time of intervention (whether ? 6 mo or < 6 mo). Significant differences were not found. The evaluated studies presented a high degree of heterogeneity and low methodological quality in the carried out analysis. CONCLUSION: Silymarin minimally reduced, but without clinical relevance, the serum levels of ALT and AST. It is necessary to carry out studies with more appropriate methodological designs.

  25. Bernice Katzman says:

    I have a 13 yr old rescued a, Yorkie that I adopted at age 4 months. She had a liver shunt which was corrected at 6 months. No issues except the last year she had multiple UTI. Last August she was diagnosed with Kidney disease. I put her on a home cooked diet from a nutritionist. and values increased and was hospitalize. Now an occurring hepatic encephalopothy. Hospital put Heidi on a boat load of meds which appeared to be making her worse. They also had her on Aternagel which for 10 weeks only saw a great increase in phosphorous level until I saw the ingredients. I reached out to a compound pharmacy and with their AL(OH)3 within 4 weeks levels dropped to less than half. Reformulated anther diet which was a big disaster and Heidi wound up in the hospital again. OI then tweaked the diet, and am just using the Aternagel, lactrulose, balance it and Noric natural fisl oil. The Welcatin made her vomit. The nutritionist told me to give her Azodyl and milk thistle which I have done this week but half the dosage. What do you think?

  26. Bernice Katzman says:

    No, signs of hepatic encehalopothy, but now worried about milk thistle and Azodyl after reading comments. Also, Heidi is given 100 ml sub-q every other day.

  27. skeptvet says:

    There is absolutely no legitimate evidence that Azodyl does anything, so I would be surprised to see it recommended by a board-certified nutritionist. Milk thistle has some protective effects on the liver in certain situations, but we don’t have much research data, so whether it will be helpful in your dog is hard to predict. Neither is likely to be significantly harmful.

    Good luck!

  28. Angie says:


    Is there any recent information on milk thistle? Would you have time to research and write another article regarding this subject? Thanks so much

  29. skeptvet says:


    Research on milk thistle seems to have gone out of style. I found a review from 2013 which covered the same studies I looked at for the original post. I have only found one study since the, in a rather obscure journal, so I don’t have much to add to the previous conclusion- some theoretical evidence it might be helpful in some situations but almost no studies in dogs or cats with natural disease.

    Prophylactic and therapeutic effects of silymarin on acute hepatotoxicity due to administration of mebendazole in dogs.
    Iranian Journal of Medicinal and Aromatic Plants
    2013 28 4 594-603

    In the present study, effect of silymarin was evaluated as a protective drug of liver against acute hepatotoxicity due to administration of mebendazole in German shepherd dogs (mixed breeds). Twenty five healthy dogs were randomly allotted to five equal groups. Dogs in group A were given mebendazole with single dose 150 mgkg, p.o.; group B consisted of dogs that received silymarin with single dose 30 mg/kg, p.o. concurrent with mebendazole administration; groups C, D and E were treated like group B, but silymarin was administered 2, 12 and 24 h after administration of mebendazole respectively. The serum concentrations of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), lactate dehydrogenase (LDH) and total and direct bilirubin were measured before administration of mebendazole and 2, 12, 24 and 72 h later as indices of liver injury. A single oral administration of mebendazole significantly elevated serum concentrations of ALT, AST, ALP, LDH in all cases of group A (P<0.05), after 24 h. In both groups of B and C, levels of serumenzyme activities remained within the normal values. The difference was significant between groups B and C with group A (P<0.05). Levels of serumenzyme activities were higher than normal values in three cases of the group D and in all dogs of the group E. This study showed that silymarin could protect liver tissue against oxidative stress in dogs with mebendazole intoxication particularly in the first 2 hours after exposure.

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