Some cats react better to the insulin if it's injected into the flanks. There have been anecdotal reports of the insulin not being absorbed as well when injected in the "scruff" or shoulder blade area. On the other hand, plenty of people inject the insulin in the scruff with no problems.
I'm copying this information from the Health Links Index sticky, specifically from Marje and Gracie's
Testing and Shooting Tips
Shooting
We all have our own techniques for shooting but just a few thoughts and others may bring some of their ideas in as well.
You can shoot the scruff, side of chest, side of belly, side of flank....wherever your kitty is comfortable and you have the least chance of doing a fur shot.
It's actually a great idea to rotate sites in order to prevent development of a lipoma or scar tissue. A lipoma is just a thickened area of fatty cells right under the skin. If you shoot in the same place repeatedly and your kitty develops one, they often will go away on their own if you switch to another site. However, if you change injection sites every day (if possible), there is a much decreased chance in developing either a lipoma or scar tissue. Both of these can affect absorption negatively. If your cat prefers scruff shots only, you can shoot one side in the morning and the other side at night; you can also move the scruff site so the first day, you shoot at the inner scruff on one side, the next day, the middle scruff, and the third day, the outer part of the scruff. Or, if kitty is tolerant of any injection sites, you can shoot one side in the mornings and shoot the scruff the first day, the chest the next, the flank the third day. In the evenings, you can shoot the other side of the kitty but in the same order.
Theoretically, shots absorb slower when given in the scruff area as opposed to the chest, belly, or flank. This does not mean the BG numbers will be different. It just affects the absorption rate....faster or slower. But, ECID and you can keep track of where you shoot and see if the absorption rates vary for your kitty depending on shot site.