Nearly all trials have concluded that glucosamine is safe.  The rate of side effects from glucosamine is similar to side effects seen with placebo.  The Cochrane Review, which looked at 16 randomized controlled trials, reported adverse side effects in only 14 study participants.  The majority of complaints were gastrointestinal, such as abdominal pain, bloating, and diarrhea.  With such a good side effect profile, few people stopped taking glucosamine and the drop out rate for all trials has been very low.

Animal data suggested that high-level parenteral glucosamine exposure could lead to insulin resistance.  A recent trial published in the Archives of Internal Medicine followed Type II diabetics taking 1500mg glucosamine hydrochloride a day.  There was not a significant difference between the baseline and follow-up hemoglobin A1c levels.

Diabetic patients should discuss glucosamine supplementation with their physician before beginning a regimen.

Because glucosamine is derived from chitin in crustacean shells, some researchers suggest that patients with seafood allergies should avoid glucosamine.  No one has published a case report of such cross-reactivity. 

Because the Dietary Supplement Health and Education Act of 1994 limits federal regulation of supplements, including glucosmaine, there is no way to assure product quality.  Consumer Reports has tested several brands of glucosamine and found the glucosamine content to be reliable.