Blunt needles: Avoid by using a new pen needle or syringe for every injection.
Wrong needle size: Always choose carefully. Insulin should be injected into the subcutaneous layer of fat just under your skin. A needle that is too long can inject into the muscle. On the other hand, using a needle that’s too short will sting, as the insulin could be injected into the skin rather than under it.
Injecting a large dose of insulin: If this is causing your bruising, your doctor or educator may recommend splitting your insulin dose into two smaller injections.
Not rotating injection sites: Give your skin a chance to heal by changing sites every few days.
Incorrect technique: The ideal technique for injecting insulin is to swab the area, pinch the skin upwards, inject at a 90-degree angle (or 45 degrees if you have low body fat), depress the plunger slowly, count to 10, remove the needle and press firmly on the injection site for a few moments. If you do see blood after injecting, place your finger on the spot fairly firmly for at least a minute to limit the spread of the bruise.