Plasma and urinary concentrations of synephrine in horses following controlled oral administration

Synephrine is a sympathomimetic alkaloid of regulatory interest in both horse racing and equestrian sports, where feed‑related exposure may lead to its detection in equine blood and urine. This study characterized plasma and urinary concentrations of synephrine in horses following controlled oral administration. Six Thoroughbred horses received single oral doses of 100 mg or 800 mg synephrine, and blood and urine samples were collected sequentially. Each sample was analyzed both before and after β‑glucuronidase hydrolysis, demonstrating that synephrine was present predominantly as conjugated metabolites in both plasma and urine, and that hydrolysis substantially increased measurable concentrations. Non‑compartmental analysis was used to determine selected pharmacokinetic parameters, including half‑life, Cmax, Tmax, and AUC. These pharmacokinetic results indicated that synephrine was rapidly absorbed, with dose‑related increases in both systemic and urinary exposure. The findings provide initial quantitative information on circulating and urinary synephrine following defined oral exposure in horses. The concentration ranges and detection patterns described here may assist in interpreting field detections and support future studies aimed at understanding exposure arising from feed‑borne sources.