Equine supplement use is frequently recommended to address common health and performance outcomes. Supplementation is commonly accompanied by belief and placebo effects, which also drive use, but may confer anti-doping and horse welfare risks. An online survey was created to assess supplement belief effects in equestrians and related non-veterinary professions. Topics included participant demographics, and belief effects regarding supplementation for gastrointestinal, joint, exercise performance, temperament and vitamin and mineral outcomes. Participants ranked seven images of supplement forms to reflect perceived efficacy for health/performance outcomes, and reported on current or prior use of supplements for each outcome. Data were analysed by Friedman’s and Kendall’s W tests for supplement factors; potential differences in perceived efficacy between rider level were assessed by Kruskall Wallis tests with Dunn’s post-hoc and Holm-Bonferroni corrections. Participants reported high supplement use across all reasons for supplementation (85.0 to 98.1%), with the exception of exercise performance (61.4%), which elite riders most commonly supplement for compared to other levels (P=0.014-0.044;d=0.58-0.99). Despite significant post-hoc differences in supplement form comparisons, Kendall’s W values were trivial to small suggesting a lack of concordance in belief regarding supplements. Current and prior supplement use is high in the equestrian population, irrespective of rationale. Supplement use for joint health and exercise performance vary significantly by participant level. A joined up approach to supplementation testing is required, regardless of form, given the high prevalence of supplement use, varying rationales for supplementation and the potential for adulteration and contamination.