This study aims at measuring gait ataxia associated with opioid intake. Patients with knee OA and age matched healthy volunteers will be recruited and are required to meet for 3 all-day assessments. Measurements of gait ataxia (gait variability) will be recorded in the gait laboratory before and every hour for 6 hours after intake of two different kinds of opioids or placebo. The order of opioid/placebo is randomised. Further, the autonomous regulation of pulse and blood pressure is assessed every hour together with self-reported pain and dizziness, and a blood sample to estimate plasma concentrations of the active opioid.