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Abstract Background Osteoarthritis is a leading cause of pain and retirement in athletic horses. Hydro-expansive functionalized nanogels, acting as Drug Delivery Systems, constitute one of the current therapeutic prospects. These nanogels have the potential to combine mechanical benefits through polymers with the biological effect of prolonged release of bioactive molecules. The purpose of this double-blinded randomized tolerance study versus negative control was to evaluate the response of healthy joints to a single injection of the expected efficient dose (further referred to as the trial dose) and overdose of nanogels composed of chitosan and hyaluronic acid and featuring a type A endothelin receptor antagonist and a type B1 bradykinin receptor antagonist. The metacarpophalangeal joints of 8 healthy horses were randomly injected with 2.4 mL of functionalized nanogels and 2.4 mL of saline as control on the contralateral limb. Injections were repeated twice at one-week intervals, followed by injection of a triple dose of nanogel on week four. Clinical, ultrasonographic and synovial fluid cellular and biochemical follow-ups were performed up to three months following the first injection. Results No change in general clinical parameters, lameness or sensitivity to passive flexion of the fetlocks was noted. Mild to moderate synovitis was noted on the day following injection in the treated group, with a significant difference (p