I switched brands after trying another N-acetyl glucosamine supplement as the last ones didn't help much with my knee pain. I found that while I was taking these my knee pain decreased slightly, but it may have been due to a diet change. I had no negative side effects from these tablets.
This stuff is great, it has helped me so much over the past year where other brands have failed. I think more people would be trying it on Iherb if it was categorized properly (it doesn't come up in a search for "NAG" or "N Acetyl Glucosamine nag"). If you've tried other brands without much success, give the Nutricology brand a go, it really works!