Glucosamine is a substance found in the body's cartilage and has long been used in supplements for dogs with osteoarthritis. It sounds biologically plausible that such a supplement could help the joint, but a clinical effect has to be shown in studies in dogs, not just assumed from the substance's role in cartilage.
Veterinary reviews have described limited and varying evidence for a long time. A systematic review and meta-analysis from 2022 found a clear lack of clinically relevant pain-relieving effect for combinations of glucosamine and chondroitin, compared with stronger support for certain other nutrition-based interventions. This does not mean that every dog responds in exactly the same way, but glucosamine should not be seen as a substitute for a proper osteoarthritis diagnosis and a broad treatment plan.
With osteoarthritis, the vet often combines weight control, adapted activity and rehabilitation with medicines or other treatments based on your dog's needs. Always say which supplements your dog is getting, so that the whole treatment can be assessed together.