This article has not yet been professionally approved. The information is based on the sources listed below.
Milk thistle for horses: what the studies on the liver show
What is known about milk thistle in horses: poor absorption of silibinin, good tolerance in healthy horses, no studies in liver disease, and what to check before competitions.
The short answer
Milk thistle is mostly fed to horses for the liver, but such a benefit is not proven. In one study, only 0.6 per cent of its compound silibinin was absorbed from the feed 1. There are no studies in horses with liver disease. Abnormal liver values should be investigated by your vet. Before competitions, check the feeding in the ADMR database 2.
What milk thistle contains
In horse studies, milk thistle (Silybum marianum) was fed, for example, as press cake from the seeds 3. One main compound is silibinin 1.
For humans, the EU medicines agency lists milk thistle as a traditional herbal medicinal product for digestive complaints and to support liver function, after serious illness has been ruled out by a doctor 4. This is a classification for humans and not evidence in horses.
What has been studied in horses
In horses, there is one study on absorption and tolerance as well as small feeding studies with lab values.
- Absorption: only 0.6 per cent of the silibinin from the feed reached the blood, 2.9 per cent via nasogastric tube 1.
- Tolerance: up to 26 mg per kg body weight twice daily for seven days, the horses showed no dose-limiting toxicity; a liver enzyme (GGT) was slightly above the reference range in three of five horses 1.
- Feeding: in a Czech study with ten horses, some blood values such as AST and cortisol differed after exercise. The groups were very small and also differed in age and ration 3.
| Study | Result |
|---|---|
| 9 horses, absorption | 0.6% from feed in the blood |
| 5 horses, 7 days | no dose-limiting toxicity |
| 10 horses, 56 days | some blood values different after exercise |
Studies in healthy horses; none of them examines horses with liver disease.
If the liver values are abnormal
Raised liver values have many possible causes. One of them is poisonous plants such as ragwort, whose damage develops over weeks to months 5. Finding the cause is a job for your vet before you change feed or supplements.
In humans, hypersensitivity to plants of the daisy family (Asteraceae) is considered a contraindication 4. Whether this applies to horses has not been investigated.
Milk thistle in competitive sport
Silibinin, silymarin and milk thistle are not named on the 2025 FEI list. However, the list also covers substances with a similar structure or effect 6. The FN also bans phytotherapeutics in competition in general 7.
So check milk thistle in the ADMR database before every competition and agree a possible withdrawal period with your vet 2.
Key takeaways
Frequently asked questions
Does milk thistle help the horse's liver?
That is not proven. Healthy horses were studied; the authors explicitly call for studies in horses with liver disease 1. Raised liver values need your vet to look for the cause.
How much milk thistle should a horse get?
There is no proven dosage. In the tolerance study, healthy horses received 6.5 to 26 mg silibinin per kg body weight twice daily for seven days, without dose-limiting toxicity 1. This is not a recommendation.
Sources & studies
The content is based on the references listed below. It does not replace individual veterinary or feeding advice.
Show list of sources (7)
- Hackett ES, Mama KR, Twedt DC, Gustafson DL (2013): Pharmacokinetics and safety of silibinin in horses. American Journal of Veterinary Research 74(10):13279 healthy horses: 6.5 mg silibinin per kg body weight intravenously, orally in feed and by nasogastric tube. Then 5 horses given 6.5, 13 and 26 mg/kg twice daily for 7 days each. Oral bioavailability 0.6% in feed and 2.9% by tube, no accumulation, no dose-limiting toxicity. GGT slightly above the reference range in 3 of 5 horses at high doses. Healthy horses – no efficacy data for liver disease
- FN – Anti-Doping und Medikation (ADMR-Suchmaschine)Official database of the FN (German Equestrian Federation) on substances and withdrawal periods in competitive sport
- Dockalova H, Zeman L, Horky P (2021): Influence of Milk Thistle (Silybum marianum) Seed Cakes on Biochemical Values of Equine Plasma Subjected to Physical Exertion. Animals 11(1):21010 Czech warmbloods (5 trial, 5 control), 56 days of up to 2 kg barley pellets containing 20% milk thistle press cake (up to 16.6 g silymarin/day), followed by 2 h of driving exercise. Trial group with lower AST, NEFA and cortisol after exercise. Very small groups; the groups also differed in protein, fat, age and sex. Laboratory values – no clinical endpoints
- European Medicines Agency, HMPC (2018): European Union herbal monograph on Silybum marianum (L.) Gaertn., fructus. EMA/HMPC/294187/2013Human medicinal monograph, adopted on 5 June 2018. Traditional use for digestive complaints and to support liver function after serious illness has been ruled out by a doctor. Contraindication: hypersensitivity to plants of the daisy family (Asteraceae). In case of jaundice or discolouration of urine or faeces, seek medical advice immediately. Side effects: mild gastrointestinal complaints, allergic reactions. Not transferable to horses
- Bildfell R (2022): Pyrrolizidine Alkaloidosis in Animals. Merck Veterinary ManualSpecialist overview, full review April 2022, updated September 2024: cattle, horses, farmed deer and pigs are the most susceptible. The liver shows the cumulative, progressive effects of repeated small doses; hepatocyte enlargement (megalocytosis) is irreversible and pronounced in horses. Poisoning also via hay, silage and pellets; at least some of the toxins survive drying. Symptoms often appear only weeks to months after intake begins, even after it has ended; in horses mainly hepatic encephalopathy (e.g. head pressing). Animals with clinical signs rarely recover. Senecio jacobaea contains jacobine
- FEI (2024): 2025 Equine Prohibited Substances List. Fédération Équestre Internationale, Lausanne, Stand 25.11.2024Version checked: 2025 (the 2026 version could not be accessed on 18 October 2026). Harpagoside is listed as CONTROLLED (analgesic), valerenic acid as CONTROLLED, capsaicin as BANNED. Silibinin/silymarin, Vitex constituents (agnuside, casticin), nettle, rosehip and ascorbic acid are not listed by name. Catch-all clause: substances with a similar chemical structure or similar biological effect are also prohibited. The list is revised annually; the current version always applies
- Deutsche Reiterliche Vereinigung (FN): Anti-Doping- und Medikamentenkontroll-Regeln für den Pferdesport (ADMR) mit Verbotsliste Anhang I–IIIRulebook with its own FN prohibited list (Art. 4.1). Annex II (prohibited medication, banned in competition) explicitly names “Harpagophytum Procumbens (Teufelskralle)” (devil's claw) under the musculoskeletal system and, under “Darüber hinaus sind verboten” (also prohibited), lists in general terms “Phytotherapeutika, wie z.B. Arnika, Ingwer” (phytotherapeutics such as arnica, ginger). Catch-all clause for substances with a similar structure or effect. Responsible persons are personally liable for everything administered to the horse (Art. 2.1.1), including with contaminated feed supplements (commentary on Art. 10.4). The document bears no date