SummaryBackgroundHerbal supplements containing curcumin and other ingredients are used for pain management in horses with osteoarthritis (OA).ObjectivesTo test the effects of a herbal supplement containing curcumin and other ingredients in horses with lameness due to naturally occurring OA.Study designTwo‐period randomised crossover design.MethodsTen Thoroughbreds with naturally occurring chronic OA were randomly assigned to the treatment (BLP; Buteless® Performance) or control (CTR, no supplement) groups and fed daily for 30 days. On Days −1 (before treatment), 15 and 30, lameness examination, range of motion, pain on palpation and force platform data were collected. Plasma curcumin concentration and its metabolites were measured on Days 1 and 14. Gastroscopy, a complete blood count and a serum biochemistry panel were performed on each horse before treatment Day −1 and Day 31. Gastric lesions (ulcers) were scored in real time by a masked investigator.ResultsMean peak vertical force (PVF), measured by the force platform, significantly increased in the lame limb of the BLP‐treated horses on Days 15 (0.40 ± 0.13 N/kg, (p = 0.0025) and 30 (0.63 ± 0.14 N/kg, p < 0.0001) compared to the CTR group. In addition, mean normalised PVF was higher in the BLP group on Day 15 (p = 0.0438) and on day 30 (p = 0.0003) when compared to CTR horses for the same days. The PVF significantly improved (≥5%; range, 5.2–33%) in six of nine individual BLP‐treated horses and did not improve (<5%; range, 0–3.4%) in three of nine BLP‐treated horses. Also, PVF improved (≥5%; range, 7.6–15.4%) in three of nine horses in the CRT group. Squamous gastric lesion scores significantly decreased in both groups by Day 31. Plasma curcumin‐O‐sulphate concentrations (1.2–3.3 ng/ml) were present in 9/10 BLP‐treated horses by Day 14.Main limitationsSmall sample size and absence of a positive treatment (non‐steroidal anti‐inflammatory drug) control.ConclusionsThe BLP supplement containing curcumin achieved plasma concentrations and improved weight bearing in some treated horses with naturally occurring OA.