There is no clinical proof that joint supplements do a darn thing. That being said I've had my horses and dogs on NutraMax products (Cosequin, Dasuquin) for years and firmly believe they help. I finally put myself on their human product (I don't care for flavored tabs) Cosamin and it made me feel a lot better. Been plagued with aggravating back pain. Now I don't take my Rx meds hardly at all.
NutraMax guarantees their product contents. It's not FDA approved so anything can go on the bottle. A vet told me start with Cosequin if it works, you can switch to a cheaper brand and if it doesn't work you know it's ingredients are not right. Also cheaper brands a larger pills that need to be administered multiple times a day. After a loading period, Cosequin is once a day.
Healthy Pets.com sells it the cheapest I have seen including shipping. Entirely Pets is the same site just also sells Rx stuff Healthy pets does not.