Insulin has an important and vital role in control of blood glucose for all diabetic patients. With increasing the prevalence of diabetes mellitus, insulin consumption will be increased too. Different types of insulin are available, i.e short, rapid, intermediate and long acting insulin and also mixed insulins. The side effects of insulin may lead to the lack of a desirable control of blood sugar or discontinuing insulin injection by the patients. On the other hand sometimes the side effects of insulin can be serious and fatal. Therefore, information about these complications and how to deal with them are very important. Hypoglycemia, weight gain, antibody-mediated insulin resistance, lipodystrophy (atrophy or hypertrophy), edema, postural hypotension and Allergic reactions to insulin are major side effects of insulin therapy.