Although supplement contamination by a prohibited agent resulting in an anti-doping violation has been extensively reported in human athletes, similar scenarios are seldom observed in equine sports. The authors report the case of a horse competing at the Paris Olympic Games where trazodone, an antidepressant, was found in the blood. Due to its sedative properties that could alter performance, trazodone is listed as a prohibited substance in horses. As the decision of the Tribunal of the Fédération Equestre Internationale about the case is now public, it is possible to share the investigations. Trazodone was identified in a liquid supplementary feed (RELAX PRO) after dilution in methanol and ultra-sonication. The drug was identified with liquid chromatography coupled to tandem mass spectrometry using 3 ions transitions. In 3 separate aliquots, trazodone was identified at 97, 97 and 102 μg/mL Criteria of identification (retention time, ratios between the transitions) were met. A segmented hair test of the tail collected from the horse (0.4 pg/mg) confirmed incidental exposure. The period of ineligibility imposed on the person responsible was 5 months while the horse was suspended for 2 months. The team results of all the country team were disqualified. The manufacturer of the supplement confirmed that it was an unfortunate contamination which occurred at the production level, resulting from a contaminated magnesium raw material.