Herbs for horses – St John’s Wort

Herbal expert James Hart takes a look at some of the most useful herbs for natural horse care

Saint_johns_wart_flowersSt John’s wort is a herb with a long but sometimes stormy past that has come into to its own as a safe and effective therapy for a thoroughly modern condition. St John’s wort (Hypericum perforatum) is native to the woods and hedgerows of Europe, western Asia and northern Africa. It is now a common weed throughout the world in tussock grasslands, pasture, roadsides, riverbeds and waste places.

It thrives in New Zealand and can be found throughout the country. Here, of course, it will flower in December with its yellow blooms. It is quite easy to identify by holding a leaf up to the light. With careful inspection you can see what look like small holes in the leaf. They are actually oil glands which allow the light to shine through. The oil is red, which gives St John’s wort oil its characteristic colour.


St John’s wort is one of the most useful medicinal herbs of the many available to us, and one that’s easy to process at home – as well as to cultivate or to find growing wild. About a year ago we found a small, weak-looking plant growing on the edge of a paddock. We put it in the garden and with a bit of TLC it is now thriving and seeding everywhere. For use with horses, it is invaluable as both a mood-enhancer and an anti-viral.

Happy pills for horses

St John’s wort is classed as a psychoactive, but we’re not talking hallucinations – just activity in the brain. A psychoactive substance is one that crosses the blood–brain barrier and acts primarily upon the central nervous system, where it affects brain function. Some of these substances are not so good, but others such as St John’s wort can result in beneficial changes to improve mood and behaviour. 

Substances of this kind may take a day or more to show their effect, and, as you would expect, the time will vary between individuals.

In humans, St John’s wort is a safe and effective antidepressant with few side effects – in fact, in Germany it is prescribed seven times as often as Prozac.

We do not often deal with depressed horses, although we have given St John’s wort to a mare who was very distressed over the loss of a foal, and it really helped her. There is also the potential to help mares and foals during the weaning process.  

In everyday use, St John’s wort can help in a range of situations where a longer-term solution to behavioural problems is needed.

Will it work for every horse?

It’s not always easy to be sure of the ideal approach for behavioural issues, but we’ve found that valerian is best in the nervous type, whilst St John’s wort is best for those horses who are being uncooperative rather than genuinely anxious or just over-excited. In these belligerent types, it can be remarkably successful.

We have a regular client who is a professional event trainer and also breaks in sporthorses and thoroughbreds for racing. She recently had an especially difficult gelding. He had been unruly, throwing himself on the ground and being totally uncooperative. He was given 20ml of a 1:2 extract twice a day for the breaking period of about eight weeks. As he learned what was required of him, the dose was progressively reduced: he is now well behaved and working nicely without his herbal support.  

It is interesting that this client also commented that if she’d had access to St John’s wort earlier in her career she could have developed a lot of talented, but mentally inadequate, horses into cooperative and effective competitors.   

We have also been supporting several horses in forced box rest due to long-term musculoskeletal issues. Some have been confined for more than six months following surgery. A mix of valerian and St John’s wort works really well here. They are on a regular 20ml twice a day of St John’s wort, to which is added valerian before they go out for their limited exercise. Without this cocktail they are literally climbing the walls.

Antiviral activity

collecting st johns wortA less widely acknowledged but equally impressive use for St John’s wort takes advantage of its anti-viral activity. A 1988 study showed it has potent activity against retroviruses, and further research has shown that the activity extends to all envelope viruses, including a wide range of herpes species.

In an acute viral infection, the immune system can be stimulated with large doses of the appropriate herb to help overcome the infection, but the use of herbs which are specifically anti-viral, and help to either block the virus from entering the host cells or prevent it from replicating, provides a more targeted approach.

We have used this herb extensively in equine herpes virus with very good success.   

Interestingly, the anti-viral activity of St John’s wort against both viral and retroviral infection is enhanced by exposure to light. Hence herbalists have always recommended sun exposure if you are taking St John’s wort. 

We have been using a formula containing St John’s wort for many years, but the most spectacular demonstration of its success was on young milking goats. During the autumn, young replacement does aged about six or seven months are particularly vulnerable to a rapid-onset viral pneumonia (species unknown). The herd in question usually lost up to 12 goatlings a year and once contracted, the condition was fatal in 100% of cases, regardless of veterinary treatment; usually within 24 hours of the first evidence of symptoms.

After some experimentation with dose rates, we gave any sufferers 20ml four times daily of a formula based around St John’s wort. Given the weight of the animals of about 15-20kg this was a huge dose, but it reflected the acute nature of the illness. The effect was nothing short of miraculous with all sufferers making a full recovery.

Treating horses doesn’t call for such big doses but we have found that it is necessary to give 40ml of the formula if the condition is acute. It is usually only necessary to treat for a few days, but this obviously depends on the specific case.

St John’s wort has also proved effective in certain types of head-tossing.  After a herpes infection, a persistent neuralgia can remain. In humans this is commonly seen in shingles (Herpes zoster) and can be distressingly painful. It seems that the same happens in the nasal passages of the horse. During and after exercise there is increased mucous production which runs down the nose over the sensitive area; this causes pain and head-tossing results.  

St John’s wort in combination with herbs to help the pain can be very successful, but treatment can take a few months. This is because herpes viruses can enter a dormant state in the nerve ganglia where no treatment is effective.    

Important things to know

1. Photosensitivity

St John’s wort has been shown to have very low toxicity but a photosensitivity condition called hypericism has been recorded in animals. This is a state of increased sensitivity to sunlight in unpigmented skin following ingestion of large amounts. It also causes increased sensitivity to handling and temperature changes, and depression of the central nervous system. The same things can occur in humans taking high hypericin extract.

However, the important thing to understand is that this reaction is seen only when the amount of St John’s wort consumed is an order of magnitude above the therapeutic dose range. One study in calves showed that the dose required to cause photosensitivity was 30-50 times greater than the therapeutic dose.

Hypericum is often noted in lists of potentially poisonous plants because of the photosensitivity issue. If it occurs as a weed infestation and is then grazed by cattle or sheep, sensitivity is likely to occur. This is only because they have had too much.  

We have been using St John’s wort on horses for many years and have been advised of only one instance of photosensitivity. This occurred on a grey horse in mid-summer and as soon as the hypericum was withdrawn there was complete recovery.

2. Herb-drug interactions

Because of its mode of action, St John’s wort is the one herb that is most likely to interact with prescribed medicine. In the human world these are mainly the antidepressant drugs, but care must be taken with several other drug families. In the equine world, drugs of this type are not commonly used, so the issue is less likely to emerge. However, it is something to be aware of if your horse is under veterinary treatment, so be sure to advise the vet – and, if necessary, check on-line or with a herbalist to see if any problem is likely. 

3. Use in pregnancy

Again we need to look to human data for guidance. Studies show that St John’s wort causes no increase in frequency of foetal malformation in humans. These results are reflected in animal studies.  

So, while St John’s wort is considered to be safe in pregnancy and lactation, it is best to give no medicine of any type during pregnancy, unless really necessary.

St John’s wort is one of the most useful herbs in common use, but if you are used to the quick response achieved with valerian you will need patience.  To achieve its calming effect, the active compounds in St John’s wort have to accumulate to sufficient levels in the brain. This can take a few days, but the anti-viral activity takes place at the cellular level, so response here is much faster.  

  • This article was first published in the March 2012 issue of NZ Horse & Pony magazine