Injecting any insulin at the same site repeatedly over time can cause a lipodystrophy: either lipoatrophy or lipohypertrophy. Either makes absorption unreliable, and can possibly lead to hypoglycemia  or hyperglycemia because insulin is not absorbed well from thickened tissue areas. But varying the injection site can cause variability in action profile, too. This BD Diabetes page  illustrates the most common areas humans with diabetes inject insulin and explains how absorption differs in various areas of the human body. This is true for ALL insulins. 
Rotating injection sites can be helpful in avoiding/eliminating these. At times, the injection technique is the cause and needs changing.
Vary Injection SiteEdit
The area needn't be very far from where the last shot was given--the distance of the width of 2 fingers will do fine as a measure. 
When shots are given again & again into an area of skin, the tissue becomes thicker at that point; a fairly good analogy would be the calluses people get on hands and feet. The callus skin is thicker and harder; injection areas become similar to this too from repeated shots. This thicker, harder skin doesn't let the body absorb the injected insulin as well as thinner, non-hardened areas. 
Most of us dealing with pet diabetes vary the side we give the injections in--right side mornings and left side evenings, for example. This is another help in avoiding giving shots in the same areas.
Many people give insulin shots in the scruff of the pet's neck, which is now considered to be a less than optimum choice. The neck area provides poor insulin absorption, due to it not having many capillaries, veins. etc. (vascularization).
- Other sites suggested by Dr. Greco include the flank and armpit. 
- Intervet recommends giving injections from just back of the shoulder blades to just in front of the hipbone on either side, from 1 to 2 inches from the middle of the back. 
- ↑ 1.0 1.1 Insulin-Section 6.1-Kinetics. InChem.
- ↑ 2.0 2.1 Injection Site Selection. BD Diabetes.
- ↑ Heinemann, Lutz (January 2008). Variability of Insulin Action:Does It Matter?-page 40 (4 of 9). Insulin Journal.
- ↑ Cook, Audrey (1 April 2010). Identifying the reasons behind difficult-to-control diabetes in dogs. DVM 360.
- ↑ Tips for Injecting Insulin. Joslyn Diabetes Center.
- ↑ Greco, Deborah (2010). Treating Diabetes Mellitus in Dogs and Cats. Western Veterinary Conference.
- ↑ Better Medicine E-Newsletter. Intervet (June 2006).
- ↑ Vetsulin-Preparing Insulin & Giving Injection-Page 2. Intervet.