Calming supplements for horses – what’s in them?

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"Keep calm and carry on." © Jan Milne
“Keep calm and carry on.” © Jan Milne

There are several products on the market that claim to help calm horses down. But what is in them? And more importantly, do they really work?

However well you manage horses, they are all individuals. Just like people, some horses in some situations become stressed and anxious. This may be due to genetic influences, previous unpleasant experiences or management. Once horses become anxious they may develop behaviours that can make managing them difficult, unpleasant or even dangerous for the horse and the owner, such as rearing, shying, biting, pulling, barging, etc.

So, recognising that horses do develop certain behaviours which we consider undesirable and that they are also frequently exposed to new or stressful situations and that some horses are more susceptible to stress than others, an ideal calmer should be able to reduce stress quickly, allowing horses to learn better and adapt to new situations faster.

Let’s look at some common equine calmer ingredients and see how they might work and any evidence for effect in horses.


Some magnesium calmers claim to have a “better” source of magnesium than others. This sounds good but is probably of little significance. Studies in mice suggested that supplementing with magnesium could have an antidepressant-like and anxiolytic-like (anxiety reducing) effect. But the amount of magnesium given was very high and was given directly into the body cavity by injection and not fed to the mice. A different study in mice showed that feeding a deficient magnesium diet produced an increase in anxiety.

Magnesium deficiency in horses is rare and most diets contain adequate magnesium.

Three sources of magnesium: from left, magnesium L-aspartate, Epsom Salts (magnesium sulfate), and dolomite. Magnesium L-aspartate has excellent bio-availability. Epsom Salts has good bio-availability, but can cause diarrhoea in excess. The magnesium in dolomite is not readily taken up by horses.
Three sources of magnesium: from left, magnesium L-aspartate, Epsom Salts (magnesium sulfate), and dolomite.

Magnesium supplementation can also interfere with calcium balance and lead to an increased risk of orthopaedic problems. One recent study where horses were fed 11g of Magnesium as Magnesium Aspartate (which is more bioavailable than some other commonly used forms of magnesium) did reduce the reaction speed in horses compared with a control (Dodd et al. (2015) Journal of Equine Veterinary Science). However, this equated to around 140g of the magnesium compound being fed each day – around 10 times higher than in most magnesium based supplements. In addition, it’s not clear whether the high magnesium dose affected the muscular response or was acting centrally (within the brain); i.e. was it calming the horse or was it simply slowing the horse’s response or both? Interestingly, orally administered forms of magnesium are not on the FEI Prohibited Substances list which should give an indication as to its likely efficacy at levels currently being fed.


If you are a rider or trainer who swears by calcium then you may be surprised to learn that there is no clear evidence linking calcium deficiency and abnormal behaviour (or behaviour) and or anxiety. Perhaps even more interestingly, infusing high calcium fluids into rats in one study caused them to eat more and develop convulsions. And in humans, stress has been shown to increase calcium and magnesium excretion in urine.

But that does not mean the stress is due to calcium or magnesium loss. It’s the other way around … stress can lead to calcium and magnesium loss!

Finally, a moderate number of studies have shown an improvement in behaviour or anxiety or other psychological conditions related to calcium – except this has been due to giving drugs which have blocked the uptake of calcium into the brain. Thus, if I were to speculate I would firstly suggest that feeding calcium is extremely unlikely to have any positive effect on behaviour as the amounts being fed in calcium-based calmers are relatively low and the scientific rationale for an effect is missing.

Secondly, if large enough amounts of calcium were fed, and if these did in fact cause the levels within the CSF (fluid bathing and surrounding the brain) to increase, based on available knowledge this if anything could lead to a worsening of any pre-existing behavioural condition.

Thirdly, calcium deficiency in horses is pretty rare unless they are growing, in very hard work, lactating, on low forage diets or there are other confounding nutritional factors such as high dietary phosphate intake.

Finally, calcium concentration (level) in the blood is very closely controlled. When calcium concentration starts to increase in the blood, it is very quickly reduced by the release of the hormone calcitonin. If calcium concentration in the blood falls, parathyroid hormone is released which releases small amounts of calcium from bone. Studies also show that increasing calcium supplementation results in increased calcium excretion in both faeces and urine.

Looking on the positive side, calcium is not dangerous at the amounts being fed. And the horses on these calmers will at least have strong bones and hooves. Again, not on the FEI Prohibited list.


Tryptophan is an amino acid and is another common active ingredient in equine calmers. Tryptophan is a precursor for serotonin, a neurotransmitter that has been associated with aggression, fear, stress and inhibition of aggression in a variety of animal species. Studies in horses suggest that low doses actually cause mild excitement as opposed to calming. Higher doses reduce endurance and can cause haemolytic anaemia (lowering of red blood cell count).

An Australian study into the behavioural effects of tryptophan on horses published in 2008 concluded that “Plasma tryptophan increases when tryptophan is administered at a dose used in some commercial products, but this is not reflected by marked behavioural changes in the horse”.

Again, like magnesium, tryptophan is not on the FEI Prohibited substances list.

B Vitamins

Many equine calming supplements contain B Vitamins although there are no studies in horses showing that these have any effect on behaviour. In human subjects, the US National Library of Medicine concluded there was “insufficient evidence to rate the effectiveness of” Vitamin B1 (Thiamine), Vitamin B2 (Riboflavin), Vitamin B3 (Niacin), Vitamin B5 (Pantothenate) or Vitamin B12 for stress, anxiety, depression or behavioural disorders. Vitamin B6 (Pyridoxine) is considered “possibly effective” for behaviour disorder in children caused by low serotonin levels (hyperkinetic cerebral dysfunction syndrome).

Valerian plant.
Valerian plant. © Longwood Gardens


There are many different species of Chamomile which all belong to the family of plants known as Asteraceae. There are no studies of the use of Chamomile in horses as a calming or anti-anxiety agent. A 2013 review article in the journal CNS Drugs found that there was some evidence of efficacy for chronic use (i.e. greater than one day) of one specific species of Chamomile in treating a range of anxiety disorders in human clinical trials. Doses of Chamomile are rarely if ever stated on equine calming supplements and therefore it is impossible to know how these relate to the levels in human studies, especially when raw herbs are used as opposed to extracts.


There is a reasonable amount of scientific evidence to suggest a calming effect of Valerian (Valerenic Acid; an extract from Valerian plants) in horses. However, there are no scientific published studies of Valerian or its extracts in horses, but interestingly Valerenic acid is on the FEI Prohibited substances list.


There are calmers on the market that contain “active” ingredients other than these above. At present there is no scientific evidence in the form of published, peer-reviewed studies to support the effectiveness of the majority of these active ingredients. This is desperately needed.

Supplements are far from cheap and you should ask yourself if the products you are using really do work or whether any effect is due to “the placebo effect”. That is you paid for it, you want it to work, you believe its going to work and you might even be convinced it has worked. Whether it really works or not is a different matter.

David Marlin

Dr Marlin has worked with the FEI (Fédération Equestre Internationale) and IOC (International Olympic Committee) and as a consultant to the British Equestrian Teams since 1994 and has been a member of the World Class Performance Scientific Advisory Group. » Read David's profile

2 thoughts on “Calming supplements for horses – what’s in them?

  • September 18, 2015 at 3:34 am

    I don’t agree that magnesium deficiency is uncommon, at least not here in the USA. I have been analyzing hay for many years, and there have been plenty that would fail to provide an adequate amount of magnesium if not supplemented. Add to that the fact that fast-growing grasses (as in the Spring here) tend to be extra low in magnesium, and you can definitely have a problem. Magnesium affects the nervous system (among many other things), and I have personally known a fair number of horses who would get “Spring Fever” — meaning they would get spooky and nervous in the Spring — which the owners thought was due to “rich” grass, but was in fact due to magnesium deficiency being especially bad at that time of year. Those same horses, when fed a magnesium supplement, did not exhibit the same behaviors. Call that anecdotal, but I’ve certainly seen it.

  • September 8, 2017 at 4:20 pm

    Susan, thank you for your comment. I just purchased some calming supplements to give to my horse, as spring is here and he was very fired up during last weekend’s endurance ride. I was disheartened by the article above, until reading your comment. It does generally seem that studies of horses are not particularly useful for riders and trainers. One study placed horses in an arena. Well, we ride them in the real world and this is where the differences will be seen.


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