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Glucosamine would be appropriately classified as an nutritional supplement that is popularly used in the hope that it will relieve osteoarthritis symptoms. It has inappropriately received acclaim as a "cure" for arthritis -- for which there is no evidence.
The good news is that glucosamine may improve symptoms of pain and stiffness in some patients with osteoarthritis, according to recent research studies. In one study, glucosamine was compared to ibuprofen (Advil, Medipren, Motrin, Nuprin.) Pain relief was more rapid with ibuprofen 400mg three times daily than with glucosamine 500mg three times daily; however, after 4 weeks the relief of symptoms was similar with the two treatments. (Of note, this dose of ibuprofen is not an antiinflammatory dose; higher amounts are typically given.)
Glucosamine has not been evaluated for other forms of arthritis, such as rheumatoid arthritis.
Glucosamine has been marketed as a "cartilage rebuilder." This is in part under the assumption that, because glucosamine is a component of normal cartilage, consuming it will assist in the rebuilding of damaged cartilage. There is no strong evidence that glucosamine alone, or in combination with chondroitin, is of value in rebuilding cartilage that is damaged by osteoarthritis.