The AAEP funded a study on oral glucosamine and chondroitin supplements and did find that they were beneficial when dosed appropriately. Many of the supplements on the market are not providing high enough levels of glucosamine.
So, choose the product with the highest glucosamine content you can find. You should shoot for 10,000mg for an average sized horse.
reduced cytokine-stimulated gene expression of a number of mediators of osteoarthritis pathophysiology, in agreement with previous in vitro studies, albeit at a lower concentration." --from the 2006 AAEP convention proceedings