A popular nutritional supplement used to treat osteoarthritis. Both glucosamine and chondroitin are found naturally in the joints, where they are assumed to be involved in joint repair: glucosamine is thought to stimulate the formation of cartilage, and chondroitin is believed to stimulate cartilage repair and inhibit enzymes involved in the breakdown of cartilage. The evidence is that these substances are safe and do reduce the symptoms of arthritis, especially pain.