Glucosamine is used in the medication of arthritis which is a degenerative joint disease that causes pain, inflammation and limited joint movement. Joints cartilage need water for lubrication, proteoglycans to attract and hold the water and collagen to keep the proteoglycans in place. Glucosamine has been shown to speed up production of both proteoglycans and collagen and it normalises cartilage metabolism which helps keep cartilage from breaking down.
What is Glucosamine? – http://answers.ask.com/Reference/Nature/what_is_glucosamine