Short answer: Injecting into the same spot again and again can cause lipohypertrophy: firm, rubbery lumps under the skin where medicine absorbs unevenly. Rotate in a set pattern, keep each shot at least a finger-width (about 1 cm) from the last one, check the skin before each shot, and skip any spot that feels hard or lumpy.
Repeated injections in one small area can cause lipohypertrophy, a thickened, firm or rubbery area of fat under the skin. It is common: in the large FITTER survey of insulin users, about one in three appeared to have it.
Medicine absorbs unevenly from these lumps. Insulin guidelines note that when people move from lumpy areas to healthy skin, their insulin needs often drop, which shows how much the lumps were changing absorption.
Lumps often hurt less to inject into. That is a warning sign, not a reason to keep using the spot.
Check your medicine's label for its approved sites. Some pens and drugs list only some of these.
Split the belly into four quarters around the navel: upper right, lower right, lower left, upper left. Use one quarter for a week (or for a set number of shots), moving each shot at least a finger-width from the last, then move to the next quarter in the same direction.
For daily shots, you can bring the thighs in too, for example belly one week, thighs the next. For weekly shots, simply change area each week.
If you find a lump, stop injecting there and ask your clinician to check it. Lumps can be hard to see and easier to feel.
With less fat, a long needle can reach muscle. Use the shortest needle (4 mm pen needles or 6 mm syringes) and pinch up a fold of skin if you use anything longer.
It is hard to remember which spot you used three days ago. Log the site with each dose, and be specific ("belly, lower left") rather than just "belly". Dose Track records the site with each shot and suggests the next one; see how site rotation works in the app.
See also: Site rotation in Dose Track