The best places to inject insulin into your dog are:
Some veterinarians feel that the scruff is not the best place to inject because this area is prone to the formation of lumps under the skin, and because it is easier for the owner to mistakenly inject into muscle or skin. Any of these would interfere with proper insulin absorption. Other veterinarians feel that the scruff is acceptable as an injection site if the owner uses proper injection technique.
Whichever injection sites you use, the key is to use proper pinch-up technique so that the needle goes into the fat layer below the skin - not into skin or muscle, and not through the pinch and out the other side, which just squirts insulin onto the pet's fur instead of into the pet.
It is important to rotate injection sites, because constant use of the same spot will cause scarring, which will affect how well the insulin is absorbed.
Some pet owners like to shave a 2" by 2" square on the dog's flank or side, then pick different injection sites within that square. As a memory aid, think of a clock's face inside the square. Give the first injection at Noon, the second at 1:00, the third at 2:00, until you have gone completely around the clock face. If you are giving two injections a day, you will not return to "Noon" for 6 days.