From
Examine.com:
Glucosamine is primarily sold as a joint health supplement. Studies show that supplementing glucosamine sulfate will reduce the rate of collagen (joint tissue) degradation and symptoms of osteoarthritis. Though glucosamine is comparable to acetaminophen, the reference drug for osteoarthritis, in potency, it is not as reliable.
Studies on athletes supplementing glucosamine are limited, but preliminary evidence suggests doses as high as 3,000mg of glucosamine sulfate may be able to slow joint degradation. This effect is most relevant for athletes participating in high impact sports, like running.
Though preliminary evidence suggested glucosamine supplementation could cause insulin resistance, follow up studies conclude that glucosamine supplementation does not affect glucose metabolism.
Glucosamine is very safe to supplement and its most common side-effect is flatulence. Glucosamine supplementation cannot cure osteoarthritis, but it can slow the progression of the disease.
If you check out the Human Effects Matrix at the link, it shows that repeated high-quality studies show Glucosamine has a minor effect.