What ingredients should I look for in a dog joint supplement?

Across the reviewers we read, the core trio is glucosamine, chondroitin, and MSM. petmd.com notes that Dasuquin adds ASU (avocado/soybean unsaponifiables) for extra support, while several products add green-lipped mussel and omega-3s. Community sources frequently mention the NASC quality seal as an easy filter for manufacturing standards.