Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of two polysaccharides, chitosan and chitin. Glucosamine is one of the most abundant monosaccharides. Produced commercially by the hydrolysis of shellfish exoskeletons or, less commonly, by fermentation of a grain such as corn or wheat, glucosamine has many names depending on country. Although a common dietary supplement, there is little evidence that it is effective for relief of arthritis or pain, and is not an approved prescription drug. In the United States, glucosamine is not approved by the Food and Drug Administration for medical use in humans. Since glucosamine is classified as a dietary supplement, evidence of safety and efficacy is not required as long as it is not advertised as a treatment for a medical condition. Nevertheless, glucosamine is a popular alternative medicine used by consumers for the treatment of osteoarthritis. Glucosamine is also extensively used in veterinary medicine as an unregulated but widely accepted supplement. Treatment with oral glucosamine is commonly used for the treatment of osteoarthritis. Since glucosamine is a precursor for glycosaminoglycans, and glycosaminoglycans are a major component of joint cartilage, supplemental glucosamine may help to rebuild cartilage and treat arthritis. However, there is little evidence that any clinical effect of glucosamine works this way. Its use as a therapy for osteoarthritis appears safe but there is conflicting evidence as to its effectiveness. Glucosamine is naturally present in the shells of shellfish, animal bones, bone marrow, and fungi. D-Glucosamine is made naturally in the form of glucosamine-6-phosphate, and is the biochemical precursor of all nitrogen-containing sugars. Specifically in humans, glucosamine-6-phosphate is synthesized from fructose 6-phosphate and glutamine by glutamine—fructose-6-phosphate transaminase as the first step of the hexosamine biosynthesis pathway. The end-product of this pathway is uridine diphosphate N-acetylglucosamine (UDP-GlcNAc), which is then used for making glycosaminoglycans, proteoglycans, and glycolipids. As the formation of glucosamine-6-phosphate is the first step for the synthesis of these products, glucosamine may be important in regulating their production; however, the way that the hexosamine biosynthesis pathway is actually regulated, and whether this could be involved in contributing to human disease remains unclear.