The dangers of cross-species substance use

Avoid adverse reactions and disqualifications at competition from incorrect usage of medications and supplements.

Over the last few years, we have seen sporadic cases of horses developing adverse reactions following the administration of drugs that were intended for use in other species. 

These have included:

  • severe, potentially life threatening, myositis reactions from injectable vitamin and mineral supplementations (such as B12 and selenium injections),

  • skin reactions from off label use of cattle pour-ons,

  • abscesses developing on the neck from contaminated medication that had been stored incorrectly. 

For farms that have current Restricted Veterinary Medicines (RVMs) for other species, e.g. dairy herds, or sheep flocks, it is important to remember that these RVMs are for the species specified only. Repeated use of these medications on horses, without veterinary consultation, will run the risk of these RVM privileges being taken away.

Impacts on competition

There have also been incidents of horses showing a ‘positive swab’ at competition after receiving supplements  that were licensed for other species. 

Under the Agricultural Compounds and Veterinary Medicines Act 1997 (ACVM), medications and supplements have to go through large numbers of clinical trials in order to prove their efficacy, safety and appropriate clearance times for use in the species that they are licensed for.

If a product isn’t licensed for a horse, then it is unlikely that these necessary trials have been undertaken. 

Multiple supplements and nutraceuticals are now on the market and, while most are batch tested to prove their ingredients are what has been stated on the label, a few are not. 

While many medications are tested to establish an appropriate meat or milk withholding period, the detection of prohibited substances is generally done on fewer animals and therefore a wider ‘safety margin’ is often recommended. The clearance of such substances from an animal can be variable.

Equine veterinarians have generally worked on a set of recommended detection periods or withdrawal times and advise owners and trainers on these.

One Clear Day Rule

To ensure that your horse is clear of prohibited substances, medications and topical treatments with no previously stated competition withhold times, you are advised to follow a ‘One Clear Day’ Rule. 

‘One Clear Day’ Rule: Ensuring no substances are administered in the 24 hour period before 12:01am on the day the horse is to race, trial or compete at a jumpout. This includes via oral syringe, nasogastric tube, injection, aerosol (ventilator, nebuliser or face mask), topical treatments to the skin or tissues, or a substance administered to the feed.

Both Harness Racing New Zealand and New Zealand Thoroughbred Racing recommend a clearance time of 36 hours (‘One Clear Day Rule’) prior to racing. 

Further information on medication rulings is available on relevant websites.

Extended withdrawal times

The detection of prohibited substances in racehorses and competition horses is becoming tighter, with low to no threshold in the minimum detection level. It is likely that further tightening of these rules will be seen in the future.

The New Zealand Equine Veterinary Association has recently updated the recommended withdrawal times for prohibited substances. This includes extended times for some of our routinely administered medications.

Auditing requirements

As part of stable auditing requirements, any medications administered or prescribed to horses under the care of their trainer must be documented, showing the:

  • medication

  • dosage given

  • route of administration

  • withdrawal time.

If you have any concerns regarding the use of medications in horses, or questions about competition withholding times, please contact us at VetSouth Equine.


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