Tramadol is a synthetic, centrally acting opioid analgesic. An extended-release tablet formulation of tramadol (tramadol ER) allows gradual release of the active drug, permitting once-daily administration. Tramadol ER administered once daily is equivalent in bioavailability to immediate-release tramadol administered four times daily, with prolonged absorption and lower peak plasma concentrations. Tramadol ER was significantly more effective than placebo in the treatment of moderate to moderately severe chronic pain in patients with osteoarthritis of the knee and/or hip in randomised, double-blind, placebo-controlled trials. In a flexible-dose trial in patients with osteoarthritis of the knee, the mean reduction from baseline in pain intensity scores over 12 weeks was significantly greater in recipients of tramadol ER than in placebo recipients. In a fixed-dose trial in patients with osteoarthritis of the knee and/or hip, the mean improvements from baseline in the pain and physical function subscale scores of the Western Ontario and McMaster Universities Osteoarthritis Index over 12 weeks were significantly greater in tramadol ER than placebo recipients. Common adverse events reported in patients with moderate to moderately severe chronic pain treated with tramadol ER 100-300 mg once daily were dizziness (excluding vertigo), nausea, constipation, somnolence and flushing.