Few, if any of the "joint supplements" have hard evidence, including in people, where there is more data with one suggestion being the dose of glucosamine has to be 1500mg per day and the benefit was marginal. That's not to say that the placeno effect isn't working. If an owner believes the product will work and so takes their dog out more, so building muscle, then that will support the joint more and reduce pain. I see thi swith dogs who have been on metacam, improved, so come off metacam and remain pain free until the muscle reduces again
The only effective stuff is the licensed medicines (metacam, carprofen and others) which are like ibuprofen. Stomach irritation is not uncommon, but can be controlled with eg omeprazole.