Glucosamine sulphate (often just called glucosamine) is widely recommended for sufferers of joint problems (especially osteoarthritis) and is one of the most common non-vitamin, non-mineral, dietary supplements for both humans and dogs.
Although it's not clear exactly how glucosamine works, or even if it works, most scientists believe that it helps to prevent the break down of the joint cartilage. Unfortunately, in clinical trials, results have been mixed and glucosamine's effectiveness is still in question.
Glucosamine can be found naturally in animal bones and bone marrow and so is present in small amounts in all meat-based dog foods. Nevertheless, many dog foods, especially those designed for older dogs or large breed dogs, have added glucosamine to provide additional joint support. Commercially manufactured glucosamine usually comes from the hydrolysis of the shells of crustaceans (shrimps, crabs etc) where it is particularly abundant.