The FEI has warned against using the muscle stimulant Fustex, which may contain the banned substance propoxyphene, a narcotic pain reliever.

Fustex is made by the Argentinian online seller Chinfield S.A.

Propoxyphene was not listed in product information at the time of a doping offence involving Emirati endurance rider Sheikh Hazza Al Nayhan, but was detected in a Fustex sample tested for the FEI by the Horseracing Forensic Laboratory.

Last month, the Court of Arbitration for Sport reduced Sheikh Hazza’s 27-month ban to 18 months after being persuaded of his yard’s efforts to avoid this kind of issue.

The FEI said that in future, “lack of knowledge regarding the ingredients of Fustex will not be a valid excuse in any equine anti-doping procedures.”

Earlier this year the FEI tightened up legal loopholes over supplements, highlighted in the Jock Paget and Kevin McNab case where the provider of the herbal supplement LessStressE admitted he made it on his kitchen table with no product controls.

LessStressE was deemed the source of the banned sedative reserpine, to which both riders’ horses tested positive at Burghley 2013.

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The FEI says supplements and herbal remedies can contain prohibited substances not mentioned on the label, or in different amounts to those listed, and underlines that accidental contamination will no longer let riders off the hook.

“Persons Responsible [the rider] are responsible for what their horses ingest and they are, therefore, responsible for any substance found in a sample,” said a spokesman.

“A contaminated supplement will not excuse a positive doping test.

“If in any doubt, do not give it to your horse’.”

The first person exonerated of blame by the FEI Tribunal on grounds of contamination was British endurance rider Christine Yeoman, who in 2010 traced the source of pig fattener ractopamine in her over-the-counter supplement to a feedstuffs plant in the US.