Natural Supplements for Horses: An Evidence-Based Review

By Sarah Bennett, Certified Animal Nutritionist

Important: Before adding any supplement to your horse's diet — especially if they compete — check the current FEI Prohibited Substances List and the rules of your national federation. Several "natural" herbs are prohibited in competition, and ignorance of the rules is not accepted as a defence. Always consult a qualified equine veterinarian, not a small animal or general practice vet, before starting any supplementation protocol.

Walk into any equestrian feed store and you will be confronted by shelves of powders, pellets, and liquids promising everything from calmer behaviour to gleaming coats to pain-free joints. The natural supplement market for horses is enormous — and largely unregulated. That does not mean every product is useless, but it does mean that horse owners need to approach the category with a critical eye. In this review, I assess five of the most popular natural equine supplements through the lens of the available scientific evidence, noting clearly where the data is strong, where it is weak, and where the risks outweigh any plausible benefit.

Turmeric and Curcumin: Anti-Inflammatory Promise, Bioavailability Problem

Curcumin, the active polyphenol in turmeric (Curcuma longa), has been the subject of hundreds of studies in humans and laboratory animals, many of them demonstrating potent anti-inflammatory and antioxidant effects. For this reason, turmeric has become one of the most popular supplements among horse owners seeking a "natural" alternative to non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone.

The science, however, requires careful interpretation. The central problem is bioavailability. Curcumin is poorly absorbed from the equine gut in its native form, and most studies demonstrating anti-inflammatory effects have used formulations that enhance absorption — for example, combining curcumin with piperine (black pepper extract) or encapsulating it in lipid-based carriers. Studies specifically in horses are very limited. A 2019 study found that oral curcumin supplementation in horses produced measurable plasma concentrations when combined with a bioavailability enhancer, but whether these concentrations are sufficient to produce clinically meaningful anti-inflammatory effects in living horses remains unclear.

Verdict: Promising in theory, but the horse-specific evidence base is thin. If you choose to use turmeric, use a formulation that includes a bioavailability enhancer and give it at least six to eight weeks before judging results. Do not use it as a replacement for veterinary pain management in acute conditions.

Milk Thistle: Liver Support with Modest Evidence

Milk thistle (Silybum marianum) contains silymarin, a complex of flavonolignans with documented hepatoprotective (liver-protecting) properties. In human and small animal medicine, silymarin has shown benefit in reducing liver enzyme elevations associated with toxin exposure and chronic hepatic disease. The rationale for use in horses centres on horses prone to ragwort (Senecio spp.) poisoning, which causes progressive liver damage, or on horses with elevated liver enzymes detected on routine blood work.

The equine-specific evidence is limited but biologically plausible. Horses absorb silymarin, and the proposed mechanism of action — reducing oxidative stress in hepatocytes and stabilising cell membranes — is consistent with what we know about equine liver physiology. For horses with confirmed liver disease, milk thistle is reasonable as a complementary measure alongside veterinary treatment. It should never replace proper investigation of the underlying cause.

Verdict: Reasonable adjunct for horses with confirmed liver stress. Requires a vet-confirmed diagnosis before use — do not supplement "just in case."

Devil's Claw: Effective but Banned in Competition

Devil's claw (Harpagophytum procumbens) is a southern African plant whose secondary tubers contain harpagoside, an iridoid glycoside with well-documented anti-inflammatory and analgesic properties. It is one of the better-studied herbal remedies in equine medicine. Several studies have demonstrated that oral supplementation with devil's claw produces measurable reductions in pain responses in horses with musculoskeletal conditions, with a comparable — though slower-acting — effect to low-dose NSAIDs in some models.

There is, however, an absolutely critical caveat: devil's claw is prohibited in FEI competition and in most national federation competitions. Harpagoside is detectable in urine for a significant period after the last dose, and a positive test carries the same penalties as any other prohibited substance violation, regardless of the owner's intent. If your horse competes at any level under FEI or affiliated rules, devil's claw must not be used.

Verdict: One of the most evidence-supported herbal anti-inflammatories for horses. Useful in non-competing horses with chronic musculoskeletal pain, under equine vet guidance. Strictly off-limits for competition horses.

Valerian: Calming Claims, Competition Prohibition

Valerian (Valeriana officinalis) is widely marketed as a natural calming supplement for horses, particularly for those that are anxious, spooky, or difficult to handle. The proposed mechanism involves valerenic acid and its effect on GABA receptors in the central nervous system — the same pathway targeted by some pharmaceutical sedatives.

The evidence for valerian in horses is weak. Most of the available positive data comes from rodent models, and the few equine-specific studies that exist have produced mixed results. Anecdotal reports from horse owners are common, but placebo-controlled trials are scarce. More importantly — and non-negotiably — valerian is on the FEI prohibited substance list and is also banned by many national federations. It has been detected in post-competition urine tests leading to disqualifications. Even very small doses can result in a positive test.

Verdict: Evidence base in horses is thin. If anxiety is a significant problem, the better approach is a behaviour assessment, appropriate training, and advice from an equine vet. Completely prohibited in competition. Do not use in any horse that competes or may compete.

Garlic: Controversial and Potentially Dangerous at High Doses

Garlic is one of the most frequently discussed "natural" supplements in equine circles, promoted for everything from fly repellence to immune support to respiratory health. It contains organosulphur compounds — particularly allicin — which do have demonstrable antimicrobial and antioxidant properties in vitro.

The problem is dose. Garlic contains n-propyl disulphide, a compound that causes oxidative damage to red blood cells in horses, leading to Heinz body formation and a potentially serious haemolytic anaemia. This has been documented in horses fed garlic at doses commonly found in commercial supplements. The toxic threshold is not precisely established — individual horses appear to vary in sensitivity — and some horses have developed clinically significant anaemia at doses that other horses tolerated without obvious signs.

A 2005 study published in the Journal of Applied Research in Veterinary Medicine found Heinz body formation in horses fed garlic over a sustained period, raising significant concerns about the safety of long-term supplementation. Given the limited evidence of clinical benefit and the documented risk of haemolytic anaemia, the risk-benefit balance for garlic in horses is unfavourable — especially for horses under heavy work or competition stress, whose red blood cell reserves are already under demand.

Verdict: The potential toxicity at commonly used doses makes routine garlic supplementation difficult to justify. Avoid in horses under sustained work. If you are using a garlic-containing product, discuss appropriate monitoring with your equine vet.

What Has Evidence vs. What Is Marketing

To summarise the landscape honestly: the supplements with the strongest evidence base in horses are those targeting specific, well-understood physiological mechanisms — omega-3 fatty acids (linseed/flaxseed oil) for coat quality and joint inflammation, electrolytes for working horses, and targeted amino acid supplementation (lysine, threonine, methionine) for horses with poor topline. These are not glamorous, but the data supports them.

Much of what is sold as "natural equine supplementation" relies on extrapolation from human or rodent data, anecdotal evidence, or studies funded by the product manufacturer. That does not make every product fraudulent, but it does mean you should be a sceptical consumer. Ask the manufacturer for peer-reviewed evidence. Check whether the active compounds are actually absorbed at the dose used. Consider whether the same outcome could be achieved through diet.

If you are looking for equine supplements from established, science-backed brands, browse the equine supplement range at Zooplus — including joint support, digestive aids, and coat conditioners from reputable manufacturers.

Key Takeaways

Always verify competition status before giving any supplement: devil's claw and valerian are prohibited by the FEI and many national federations — a positive test carries serious penalties.
Turmeric/curcumin shows anti-inflammatory potential but has poor bioavailability in its raw form; only formulations with bioavailability enhancers are worth considering.
Garlic can cause Heinz body haemolytic anaemia in horses at doses found in commercial products — risk outweighs benefit for most horses.
Milk thistle and devil's claw have the strongest evidence base among the supplements reviewed, but both require confirmed diagnosis and equine vet involvement before use.
Peer-reviewed, horse-specific evidence is sparse across this category — approach marketing claims with healthy scepticism and always consult a qualified equine veterinarian.

References

Pearson W, Boermans HJ, Bettger WJ, McBride BW, Lindinger MI. "Association of maximum voluntary dietary intake of freeze-dried garlic with Heinz body anemia in horses." American Journal of Veterinary Research. 2005;66(3):457–465. PMID: 15822588
Warnock JJ, Benson B. "Review of equine musculoskeletal disease and the role of complementary and alternative therapies." Journal of Equine Veterinary Science. 2019;79:62–72. PMID: 31405509

Written by Sarah Bennett, Certified Animal Nutritionist. The information in this article is intended for educational purposes only and does not constitute veterinary advice. Always consult a qualified equine veterinarian before beginning any supplementation programme for your horse.