Rhein, the active metabolite of diacerein, inhibits interleukin-1 activity. Consequently, collagenase production in articular cartilage is reduced. Rhein dose-dependently inhibits superoxide anion production, chemotaxis and phagocytic activity of neutrophils, and macrophage migration and phagocytosis. Articular cartilage damage is reduced by diacerein in animal models of osteoarthritis. Diacerein does not alter renal or platelet cyclooxygenase activity and may therefore be tolerated by patients with prostaglandin-dependent renal function. In clinical trials of < or = 6 months' duration, oral diacerein 50mg twice daily was associated with improvement in 57 to 85% of patients with osteoarthritis. Pain scores and measures of joint function were generally reduced compared with baseline and placebo. Diacerein had similar efficacy to NSAIDs, but a slower onset of action, in comparative trials of < or = 2 months' duration conducted in patients with osteoarthritis. The predominant adverse effects of diacerein are diarrhoea and related disorders.