There is very little in the way of scientific, blinded studies that will substantiate the use of G/C. It's over-the-counter use is not regulated by the FDA, and product purity can be questionable. That is a reason to use a quality brand such as Cosequin DS, especially when taking this product for a specific therapeutic benefit. Then if it works, one can shop around to see if the benefit is maintained. That is what I recommend for my patients....it can't hurt. You also need to realize that some problems will get better (or worse) no matter what you do, but again, it is a very safe product that MAY help. Consumer Reports put out a good article on this product a year or so ago.
As far as use in dogs...again it can't hurt. I have several Vets as patients and I have never found one yet that didn't use it themselves, as well as using it for their patients. I routinely use it in pups with the thought and purpose that it MAY help in developing a better quality articular cartilage, and it MAY help in repair of micro trauma repair of developing articular cartilage, which may be incurred in play and/or training, especially in a working dog. That would be my same reasoning for using the product throughout the dogs career. As far as a dosage, I don't think a mega-dose is needed, just a diet/supplement that contains the G/C for availability to the joints if needed. I don't have any doubts about the benefit for aging, arthritic dogs along with NSAIDS on an as needed basis. There is some question as to whether glucomamine bioavailability is better as a HCL, such as Cosequin DS, or as a sulfate which the large majority of companies manufacture. As I understand it, it is cheaper to make as a sulfate, hence the increase in cost of Cosequin. But maybe that is why some people experience better results? I've used both, but I don't have data to support one way or the other, and I don't think anyone else but the companies with self-interest do either.