Purpose: Glucosamine is widely used by patients with osteoarthritis (OA) to provide symptomatic relief and to delay disease progression. However, clinical studies have reported inconsistent clinical outcomes. The current study hypothesized that the reported inconsistent clinical results could be, in part, due to variable bioavailability and elimination of glucosamine. This study therefore aimed to determine steady-state minimum plasma concentrations (C-ss min) of glucosamine to examine the variability among patients taking the supplement. Methods: Patients with OA who had been taking glucosamine for at least 1 week were recruited. Patients' blood samples were collected 24 h after the ingestion of the previous dose to determine Observed C-ss min and after a 5-day washout period to determine the endogenous glucosamine levels (GlcNend). The Actual C(ss min )was calculated by using the following equation: Actual C-ss min = Observed C-ss min - GlCNend. The glucosamine plasma concentrations were determined by using a previously developed HPLC method. Findings: Ninety-one participants (age range, 42-89 years; mean [SD] age, 68.2 [7.6] years) were recruited (70% females). There was substantial (106-fold) variation, with a 45% coefficient of variation, between the Actual C-ss min levels (3-320 ng/mL) in participants. No significant association of Actual Css min was observed with various dose- and patient-related variables.