While glucosamine supplements are likely safe for the majority of people who take them, (after first consulting with a doctor) they still may experience a few adverse side effects. Some of the most common side effects include stomach ache and stomach and digestive issues such as nausea, diarrhea, constipation, gas, and bloating hair loss, and puffy eyes. Immediately stop use if you experience any symptoms of an allergic reaction (i.e. difficulty breathing, swelling of the throat, face, lips or tongue, and/or hives) or irregular heartbeat or swollen legs.