What are the side effects of insulin therapy?

A few things. The most fear side effect of Insulin therapy is low blood sugar. That's why it's very important to keep close contact with your physician if you were just started on insulin. Follow instructions and don't overdo the insulin. Other side effects: pain, injection site irritation, weight gain...
Low sugars, weight. Insulin has been a life saving medicine for almost 100 years. People with type 1 as well as long-standing type 2 require it to replace the body's missing hormone. The body secretes Insulin in an amount appropriate for food and exercise so the sugar is normal. Taking Insulin injections is an approximation so if there is too much the sugar will go low. Weight gain is also not uncommon with insulin.