Nobody is taught this properly. A pharmacist hands over a box, says “just in the tummy,” and that is the totality of the instruction most people receive before putting a needle into themselves fifty-two times a year.
The site itself barely matters. The technique around it matters more than anyone tells you.
The three approved sites
Abdomen. The default, and the easiest to pinch. Stay at least two inches — roughly two finger-widths — clear of the navel, where the tissue is tethered and more sensitive. You have a large usable area here, which is why it tolerates weekly rotation best.
Front of the thigh. The upper outer quadrant, halfway between hip and knee. Good if abdominal tissue is tender or scarred. Some people find thigh injections sting slightly more.
Back of the upper arm. Clinically fine, practically awkward — most people cannot pinch their own tricep properly, and injecting into a flat, unpinched arm risks going into muscle. Reasonable if someone else is injecting for you.
Absorption differences between these sites are, for weekly GLP-1s, clinically irrelevant. The old insulin logic about faster uptake from the abdomen does not meaningfully apply to a drug with a five-to-seven day half-life.
Rotation, and the lump nobody warns you about
Injecting the same square inch week after week produces lipohypertrophy — firm, rubbery thickening of the fat beneath the skin. It is well documented in insulin users and there is no reason to think this class is exempt.
The problem is not cosmetic. Drug injected into hypertrophied tissue absorbs erratically. You get a week that feels like nothing, followed by a week that feels like a double dose, and you spend both of them wondering what you did wrong.
A workable system:
- Move at least one inch from last week’s site every time
- Work around one region in a clockwise pattern for a month, then switch regions
- Feel the area before you inject — if it is firm or numb compared with the skin around it, use somewhere else
- Never inject into scars, stretch marks, moles, bruises or inflamed skin
Four regions rotated monthly gives you a four-month cycle before any spot is revisited, which is comfortably enough.
The technique errors, in order of how often I see them
Injecting straight from the fridge. By a distance the most common cause of stinging. Cold fluid entering subcutaneous tissue hurts. Take the pen out fifteen to thirty minutes beforehand and let it come to room temperature.
Withdrawing too fast. Both semaglutide and tirzepatide pens need the needle held in place for several seconds after the dose window reads zero — count slowly to ten. Pull out at six and a portion of the dose tracks back up the needle path and sits on your skin. That is the mysterious “wet patch,” and it is lost drug.
Rubbing the site afterwards. It increases bruising and can push drug back toward the surface. Press, do not rub.
Reusing needles. Pen needles blunt after a single use — the tip deforms visibly under magnification. A reused needle tears rather than pierces, which is precisely how bruising and lipohypertrophy get started. Use a fresh BD or Novofine needle every time, and dispose of it in a proper sharps container rather than a coffee tin.
Tensing. A tensed abdominal wall pulls the fat layer thin. Sit down, breathe out, then inject.
2 inches
Minimum clearance from the navel when injecting into the abdomen
10 seconds
Hold time after the dose counter reaches zero, to prevent leakage
1 use
The number of times a pen needle is designed to be used
Timing, and the day you want to change
The injection day is fixed weekly, but it is movable. With semaglutide you may shift your day provided at least 48 hours have passed since the last dose. With tirzepatide the gap is 72 hours.
Choosing the day itself is underrated. Nausea, when it comes, tends to land one to two days after injection. Injecting on a Friday puts the worst of that on a Saturday and Sunday. Several patients of mine moved to Thursday or Friday purely for this reason and describe it as the single most useful change they made — worth knowing before you decide whether your nausea is normal or not.
Almost everything people describe as a bad reaction to the drug turns out, on questioning, to be a bad injection.
Device problems are a separate matter from technique — clicking, stuck buttons, leaking and the question of whether you actually got your dose are covered in when the pen misbehaves.
When a site is telling you something
Mild redness, a small bruise, or an itchy bump lasting a day or two is ordinary and needs nothing.
Worth a call to your clinician: a hard lump that persists between injections, spreading redness with warmth and tenderness, a site that is still painful after 48 hours, or any fever. Those describe infection or a genuine reaction, and neither is common enough to shrug at.
The thing worth internalising
The pen is more forgiving than you think, and your technique is less consistent than you think.
Warm it, pinch it, hold it, rotate it. Four verbs, fifty-two weeks a year, and you eliminate most of what people mistake for the drug being difficult.