insulin does not harm in any way in long run. it is the hormone which is produced in the body and because of its deficiency it needs to be taken from outside. the only thing which is going to affect in the long run is blood glucose levels. if it is not maintained at good level the organs like kidney, brain, eye will be more affected in hte long run. get the HBA1c tested once in 3 months. it is the best method to see how the blood glucose is kept under control in the last three months