The most common side effect of insulin, including Lantus®, is low blood sugar (hypoglycemia), which may be serious. Some people may experience symptoms such as shaking, sweating, fast heartbeat, and blurred vision. Severe hypoglycemia may be serious and life-threatening. It may cause harm to your heart or brain.
Other possible side effects may include swelling, weight gain, administration site reactions, including changes in fat tissue at the administration site, and allergic reactions, including itching and rash. In rare cases, some allergic reactions may be life-threatening. Other people may not experience symptoms of hypoglycemia. It is, therefore, important to monitor your blood sugar levels regularly. For a full description of potential side effects please refer to the full prescribing information for Lantus®.