The person shown in this photo link below  has had diabetes since age 17. Throughout the many years she used insulin, she injected it into only the two areas which are overgrown with fatty (adipose) tissue. Insulin  itself stimulates fat storage. The photo illustrates what continuing to inject it in the same places over an extended time period can do.
Changing or rotating the site one injects insulin is a good way to prevent lipohypertrophy and lipoatrophy, as well as preventing possible absorption problems.  Insulin does not absorb well from these thickened areas.
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. 
- ↑ Nussey, SS., Whitehead, SA. (2001). Endocrinology: An Integrated Approach-Action of Insulin on Adipose (Fatty) Tissue. National Institutes of Health.
- ↑ Nussey, SS., Whitehead, SA (2001). Endocrinology:An Integrated Approach-Lipid Metabolism. National Institutes of Health.
- ↑ Insulin Therapy. Pediatric Oncall.
- ↑ Insulin-Section 6.1-Kinetics. InChem.
- ↑ Cook, Audrey (1 April 2010). Identifying the reasons behind difficult-to-control diabetes in dogs. DVM 360.
- ↑ Tips for Injecting Insulin. Joslyn Diabetes Center.
- ↑ Injection Site Selection. BD Diabetes.
- ↑ 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.