Insulin is a natural hormone, so side effect is not expected. Sometime we do see skin allergy with animal Insulin, but this is rare. There is a chance of little gain in weight, most of the cases that is due to better control (you loose weight when sugar is uncontrolled and you gain your lost weight back when your control improves). Low sugar (Hypo) can occur and that is mainly due to mismatch or miscalculation of diet and Insulin.
What is the difference between U-40 and U-100
This is the concentration of Insulin; U-40 means there are 40 units of Insulin in one ml of the solution and in U-100 100 units. Unfortunately both concentrations are available in India but steps are being taken for only U-100 to be available in future. You must make sure your syringe is of the same type, i.e., 40 or 100 capacity, otherwise you will be injecting totally different amount of Insulin and that can be dangerous. For example, if you draw U-100 Insulin with a U-40 syringe, you will be taking two and a half times more Insulin than you are intended to (if you are planning for 20 units you will be taking 50 units).
Can I not take Insulin without
We all are waiting for the day when Insulin can be given without injection. It is encouraging that plenty of research is on the pipeline to find out an Insulin delivery system without injection. But for now we need to inject Insulin.