Side effects of glucosamine may include drowsiness, headache, insomnia, and mild and temporary digestive complaints such as abdominal pain, poor appetite, nausea, heartburn, constipation, diarrhea, and vomiting. In rare cases, the combination of glucosamine and chondroitin has been linked with temporarily elevated blood pressure and heart rate and palpitations.
Since glucosamine supplements may be made from shellfish, people with allergies to shellfish should avoid glucosamine unless it has been confirmed that it is from a non-shellfish source. The source of glucosamine is not required to be printed on the label, so it may require a phone call to the manufacturer.
There is some evidence suggesting that glucosamine, in doses used to treat osteoarthritis, may worsen blood sugar, insulin, and/or hemoglobin A1c (a test that measures how well blood sugar has been controlled during the previous three months) levels in people with diabetes or insulin resistance.
Theoretically, glucosamine may increase the risk of bleeding. People with bleeding disorders, those taking anti-clotting or anti-platelet medication (such as warfarin, clopidogrel, and Ticlid), or people taking supplements that may increase the risk of bleeding (such as garlic, ginkgo, vitamin E, or red clover) should not take glucosamine unless under the supervision of a healthcare provider.
The safety of glucosamine in pregnant or nursing women isn't known.
Clegg DO, Reda DJ, Harris CL, Klein MA, O'Dell JR, Hooper MM, Bradley JD, Bingham CO 3rd, Weisman MH, Jackson CG, Lane NE, Cush JJ, Moreland LW, Schumacher HR Jr, Oddis CV, Wolfe F, Molitor JA, Yocum DE, Schnitzer TJ, Furst DE, Sawitzke AD, Shi H, Brandt KD, Moskowitz RW, Williams HJ. Glucosamine, chondroitin sulfate, and the two in combination for painful knee osteoarthritis. N Engl J Med. (2006) 354.8: 795-808.
National Institutes of Health. "Glucosamine sulfate: MedlinePlus Supplements". April 2011.