Glucosamine may cause a variety of side effects, including allergic reactions, drowsiness, cataracts, irregular heart rates and abnormal blood sugar levels. Studies show that glucosamine is likely safe when taken orally in 500 milligram doses three times daily for up to 90 days, or as one daily 1,500 milligram dose for up to six months. Evidence indicates that glucosamine may be an effective treatment for knee osteoarthritis, notes Mayo Clinic.
The Food and Drug Administration does not regulate herbs and supplements as strictly as it does drugs, so glucosamine's side effects may be inconsistent. Allergic reactions may manifest as swelling under the skin or worsened asthma. Additionally, glucosamine is made from shellfish, so those with pre-existing asthma or an iodine or shellfish allergy should not take glucosamine, warns Mayo Clinic.
Glucosamine may cause sedation or insomnia, so patients who regularly drive or operate heavy machinery should take it with caution. Additionally, glucosamine may increase the risk of bleeding, increase blood pressure, affect insulin resistance or cause dry eyes. Patients with bleeding disorders or who are on blood thinners should use it with caution, as should patients with high blood pressure or diabetes and those who are at risk for developing cataracts, states Mayo Clinic.
Glucosamine may cause a variety of less direct side effects such as anorexia, neck or back pain, heartburn, liver problems, or vomiting, lists Mayo Clinic.