There are 2 common problems that can occur at insulin injection sites:
Lipoatrophy means fat under the skin disappears and it causes shallow indentations in the skin at the injection site. This happens because of allergic reaction in which the fatty tissue is destroyed.
Lipohypertrophy means excessive growth of fat cells beneath the skin. It makes the skin look lumpy. It is a firm knob like tissue that you can feel when you touch this area.
This happens when you use the same site over and over again for injecting insulin.
Also see: Why Abdomen is the Best Site for Insulin Injection?
Image of lipohypertrophy (a) and lipoatrophy (b)
Picture of lipohypertrophy (a) and lipoatrophy (b)
In order to avoid these complications rotating the injection site is a must.
If a patient uses the same needle over and over again, occasional bleeding or bruising may occur.