Dimethyl sulfoxide (DMSO) is a natural substance (liquid and solvent) found naturally in the horse, as well as in nature and in horse feed such as lucerne (alfalfa hay) and oats.

DMSO is known to significantly increase the permeability (enhance the transport) of certain substances through the skin of the horse and it has additionally been implicated to have some anti-inflammatory effects. In the horse it can be used both topically (on the skin) and injectable (it can be administered by infusion).

Within the IFHA and at the NHA the use of this prohibited substance in the horse has historically been monitored against an International Threshold of prosecution of 15 µg/ml in URINE and 1000 ng/ml (equivalent to 1 µg/ml) in PLASMA.  While the specified concentrations have remained unchanged, DMSO is now instead controlled by International Residue Limits (IRL’s) of respectively 15 µg/ml in URINE and 1000 ng/ml in PLASMA. 

Within studies where 63 mg of DMSO was administered to horses by means of intravenous solution the detection time relative to these thresholds was about 36 hours. This was also the detection time for single topical administration of 30 g to horses. This is reflected by the official 36 hour Detection Time historically indicated on the NHA website, which remains in place.

Methylsulfonylmethane (MSM, dimethyl sulfone, methyl sulfone or methanesulfonylmethane) is a highly related oxidation product of DMSO. It is most well-known as a supplement which is claimed to aid the health of collagen, cartridge and connective tissue, but has also been implicated to assist in muscle recovery and to reduce joint inflammation.

MSM (similar to DMSO) is found in nature and in the horse. MSM is a significant metabolite of DMSO and following its administration MSM is also elevated for 36 hours.

It is active in the horse and therefore is a prohibited substance. The NHA will now adopt the recently implemented International Screening Limit (IRL) of 1200 µg/ml MSM in URINE. This will be effective 15 November 2021.

Note that MSM is contained in many supplements which are indicated for joint health and recovery. When 15 g of MSM is orally administered to horses it was shown that urine concentrations exceeded the above IRL for 96 hours (which is the official Detection Time to be applied).

It is not ideal to have both MSM supplementation and DMSO administration in the horse at the same time, as this is likely to extend the detection time and the withdrawal time.