
If you're giving glucosamine meant for humans, I believe you need to also make sure they get vitamin C to be able to absorb the nutrients, mind you, you need vit C to be able to process most things. I have a girl with slight arthritis due to an old injury, she gets human glucosamine/chondroitin tablets twice a week, and as she's raw fed, she gets plenty of vit C in her blended veggies.
Other things good for joints are turmeric, and apple cider vinegar, as well as the devil's claw and green lipped mussel extract already mentioned. I use turmeric and apple cider vinegar as part of the veggie mix, and I haven't had a problem with Indie for a good 8 months now. Another product worth giving a go is a bioflo collar, although they're not cheap; I had one for Indie, but I think because her problems stem from a soft tissue injury I didn't see much effect, where as I know others who've tried them have seen very positive results. Indie's a Labrador by the way, but I'm sure it's pretty much the same as for GSD's.