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Summary: Objective: We investigated the synovial and plasma glucosamine concentrations in osteoarthritic patients following oral administration of crystalline glucosamine sulphate at the therapeutic dose of 1500mg once-a-day for 14 days. Design: Twelve osteoarthritic patients (six males and six females) received 14 consecutive once-daily oral administrations of crystalline glucosamine sulphate soluble powder (1500mg), in an open fashion. Plasma and synovial fluid were collected simultaneously from the same patient, at baseline and, at steady state (3h after the last dose). Glucosamine was determined in plasma and synovial fluid by liquid chromatography–tandem mass spectrometry. Results: Median endogenous glucosamine concentrations in plasma and synovial fluid were 52.0ng/ml (0.29μM) and 36.5ng/ml (0.21μM), respectively (P =0.001), and varied substantially among patients (41–121ng/ml and <10–67ng/ml, respectively). Three hours after the last dose, glucosamine concentrations resulted increased from baseline in all patients with median increases of 20.5 and 21.5 folds in plasma and synovial fluid, respectively, the difference being not statistically significant (P =0.11). In plasma, the median post-treatment value was 1282ng/ml (7.17μM) and ranged from 600 to 4061ng/ml (3.35–22.7μM). The median post-treatment synovial glucosamine concentration was 777ng/ml (4.34μM), i.e., significantly lower than in plasma (P =0.001), and ranged from 577 to 3248ng/ml (3.22–18.1μM). Plasma and synovial glucosamine concentrations were highly correlated and were in the 10μM range. Conclusions: Glucosamine is bioavailable both systemically and at the site of action (the joint) after oral administration of crystalline glucosamine sulphate in ostaeoarthritis patients. Steady state glucosamine concentrations in plasma and synovial fluid were correlated and in line with those effective in selected in vitro studies. [Copyright &y& Elsevier]