There is no general answer to your question. You have to evaluate any substance as if it was a drug. You need to look into proper therapeutic dosage (do not just go by the back of the bottle). You need to check for possible interactions with prescriptions or other over-the-counter medications that you are taking. You need to make sure the supplement, herb or vitamin is not contraindicated for any chronic condition that you may have. For example, people with high blood pressure should not use goldenseal.
Because supplements are not regulated the way that prescription medications are, you need to do your own research to see if there are reputable, controlled, double-blind scientific studies (not just marketing claims) for the efficaciousness of a specific substance. Fortunately, with the Web today, that is not as hard as it once was.
To answer your specific question about glucosamine/chrondroitan. Yes, they can help. An analysis of the studies from the National Institutes of Health provides some useful data. Here is an excerpt:
“Trials of glucosamine and chrondroitin preparations for OA symptoms demonstrate moderate to large effects, but quality issues and likely publication bias suggest that these effects are exaggerated. Nevertheless, some degree of efficacy appears probable for these preparations.”
I will also add that I had recommended that my Mom begin using them for her knee pain. She is not at all a believer in alternative medicine. The form she was getting in were these really large horse pills. After a few months, she decided they were too difficult to take so she stopped. Within two weeks, she had so much knee pain she could barely hobble. Needless to say, she got it in a small capsule and started taking it again right away. She told me this story with real wonderment and said, “That stuff really works.”
Sorry about the re-injury. Good luck with your knee.