Most insulin is injected into the fatty layer just under the skin (subcutaneous tissue), not into muscle. This slows absorption to a predictable rate matched to how the insulin was designed to act.
Common injection sites
- Abdomen - generally the fastest, most even absorption; avoid a two-inch ring around the navel.
- Thighs - front and outer thigh, slower absorption than the abdomen.
- Upper arms - back/side of the upper arm.
- Buttocks - upper outer area, slowest absorption.
Why rotating sites matters
Injecting in the same exact spot repeatedly can cause lipohypertrophy - a lumpy build-up of fatty tissue that absorbs insulin unpredictably. Rotating within and between sites on a regular pattern helps keep absorption consistent.
Pens vs. syringes vs. pumps
Prefilled or reusable insulin pens are the most common delivery method today - they're discreet and dial to a precise dose. Vial-and-syringe is still used, often because it's less expensive. Insulin pumps deliver rapid-acting insulin continuously through a small catheter, adjusting the background rate throughout the day and giving bolus doses at meals.
General technique
Clean the site, pinch the skin if using a shorter needle isn't guaranteed to avoid muscle, insert at the angle recommended for your specific pen or syringe, inject steadily, and hold for several seconds before withdrawing to reduce leakage.