Question: Why Should I Rotate Where I Inject Insulin?
Rotating injection sites helps prevent damage to the skin and underlying tissue. Insulin can be irritating and cause hardening of the skin (lumps, bumps, dimpling) and weakening of fatty tissue under the skin. Over time, the thickened skin may not have nerve endings anymore. Shots may become painless as a result, but that's a sign that the skin is becoming more damaged.
But the biggest danger of continuing to inject at the same area of your body is that damaged tissue doesn't absorb insulin as easily and at the correct rate. The more damaged the skin at the injection site is, the more difficulty controlling your blood glucose levels may be.
"Insulin Injection Sites." Health Information. 04 APR 2004. University of Wisconsin Hospitals and Clinics Authority. 27 Sep 2007.