There are no clinical studies that clearly demonstrate that there are any known side effects to using Glucosamine. The same is true of Chondroitin Sulfate which is often found in glucosamine formulas.
At least one clinical study suggested that glucosamine may alter blood sugar, insulin or hemoglobin levels. However, other studies have found no such relationship between glucosamine and blood sugars and/or hemoglobin levels. Hemoglobin levels are a “measure” of how well blood sugar levels are controlled. At least in those studies involving humans, the use of Glucosamine has been found to be well tolerated by the body at least in the short term.
Although there are many symptoms that are occasionally reported from using glucosamine, it appears to be a result of over-dosing. That may include some stomach upset; nausea; constipation; diarrhea or vomiting. Again, these appear to be extremely rare. Even when reported, they tended to resolve quickly.
There have been some reports of increased risk of bleeding for those who are taking warfarin or similar medications as well as gingko biloba, garlic, vitamin E or clover. If you are taking these supplements and wish to use glucosamine you should discontinue the use of them. If you are under a doctor’s care, you should inquire about discontinuing any medication that has been prescribed for you.
In conclusion, it would appear that glucosamine is safe and effective for its intended use with very few exceptions.