Glucosamine significantly reduces risk of lung cancer and lung cancer mortality
These results from observational studies almost never replicate in randomized studies. It's because people who decide to take glucosamine (or not) also do lots of other confounding things, like visit the doctor more, or have higher salaries, or don't live with people who smoke etc. It's been shown many times you can't really adjust for these things. Epidemiologists love doing these studies, because they are comparatively easy. The hardest part is the data sharing agreement with whatever registry you are using. You don't even need to leave your desk to collect any data.