Overall injections are fairly painless, although occasionally a nerve under the skin is irritated, which can hurt but doesn’t mean anything serious.
Insulin should be given into what is called the subcutaneous (SQ) tissue. This is so the insulin can be slowly absorbed into the bloodstream. If it is given into the muscle or a blood vessel, it will act much more quickly. In general, the needles for giving insulin make sure that it goes into the SQ tissue and nowhere else.
If insulin is given at the same site over and over again, scar tissue can build up.