Hi Martin.
Skin irregularities can sometimes occur at injection sites due to changes in the subcutaneous fat, of which there are three types.
Fat hypertrophy (also known as "lipohypertrophy" or "insulin hypertrophy") appears as soft, often "grape-like" lumps at the injection sites. This unusual condition may be caused in some people by the natural effects of insulin (one of which is to cause fat to grow) or by reuse of needles. To prevent the further development of hypertrophy, rotate injection sites and don't reuse needles.
Fat atrophy (also known as "lipoatrophy") is a loss of fat under the skin's surface. This rare condition appears as a dip in the skin and has a firm texture. It occurs much more commonly with impure insulins.
Scarring of the fat (also known as "lipodystrophy") is caused when you inject too many times into the same site or when you reuse a needle. To prevent lipodystrophy:
Rotate your injection sites
Rotate where you inject within your injection sites
Rotate the sides (right, left) of your body where you inject within your injection sites
Always using a new syringe or pen needle with each injection
Some people find that it's less painful to inject into their lumps or dips (often referred to as "lipos") than it does to inject into healthy tissue. Even though it's tempting, you should never inject into lipos because insulin doesn't absorb well there; in fact, you may need to inject almost twice as much insulin into lipos as you would into healthy tissue to get the same results.
Lipos are often easier to feel than to see, so check your injection sites with your fingers frequently. If any unusual growths, textures, bumps, or indentations have formed, switch to another site and let your healthcare professional know about them.
Shorter needles can also help with the pain.
