Last month's World Equestrian Games in Tryon, North Carolina produced no doping failures among the 163 horses tested, but two positive tests for a controlled medication substance in samples taken from endurance animals.
This was confirmed by the International Equestrian Federation (FEI) whose prohibited substances list is divided into two sections - controlled medication and banned substances.
Controlled medication substances are those that are regularly used to treat horses, but which must have been cleared from the horse's system by the time of competition.
Banned substances "should never be found in the body of the horse".
The errant samples taken on September 11 belonged to Mora Inocente, ridden by Argentina's Pablo De Los Heros, which showed up the corticosteroid dexamethasone, and El Pangue Ciromagnum, ridden by Raimundo Undurraga Mujica of Chile, which tested positive for the corticosteroid triamcinolone acetonide.
These two cases will be heard by the FEI Tribunal and, as they involve only a single controlled medication substance, there is no mandatory provisional suspension of the person responsible.
Human anti-doping testing was also carried out in Tryon, in conjunction with the US Anti-Doping Agency (USADA), and produced no positives from 92 samples taken from athletes at the Games.
"Clean sport is an absolute must for the FEI and we are very encouraged by the absence of any positives for banned substances and that all human tests came back negative from last month's Games," FEI secretary general Sabrina Ibáñez said.
"While of course we cannot overlook the two controlled medication positives, overall the outcome is evidence that the awareness campaign conducted prior to Tryon, the opportunity for our National Federations to test their horses before departure and, on the human anti-doping side, our excellent cooperation with the United States Anti-Doping Agency, all had a positive impact.
"These two positives show that our testing programme works, but even though these are not doping substances, athletes should be aware that treatments from the controlled medications list must have been cleared from the horse's system by the time of competition.
"It is clear that we need to work even harder to get the message across that clean sport and a level playing field are non-negotiable."
Enhanced anti-doping measures were introduced before Tryon 2018 as part of the FEI's Clean Sport campaign, with National Federations offered two types of anti-doping testing so that they could ensure horses were clean.
These were pre-arrival testing (PAT) and elective testing.
PAT, available for both the World Equestrian Games and the Olympic and Paralympic Games, detects prohibited substances, with no limit to the number of substances tested for.
Elective testing is for controlled medications only and is limited to four substances.