I've heard both good and bad results and probably will vary by individual. I have started taking a packet of glucosamine + vitamins for breakfast as a preventative.
Because these are just sugar molecules, the only real risk seems to be to individuals who have diabetes because it can raise blood-sugar levels.
If anyone cares:
The extracellular matrix is secreted by cells and fills up all the empty space. One of the main components are long repeating sugars. Glucosamine is one of these sugars. Depending on the second sugar, how they are linked, and addition of sulfur groups determines the overall type of chain: hyaluronic acid, chondroitin sulfate and dermatan sulfate, heparan sulfate, or keratan sulfate.
http://www.ncbi.nlm.nih.gov/books/bv...c4.figgrp.3543
The mechanism of how taking these helps is unknown but there are other examples of similar outcomes (eating yogurt helps prevent yeast infections but how?). It seems like a cheap alternative for an everyday pain that has minimal side effects.