Injection site rotation is the part of GLP-1 therapy that everyone is told about once and then quietly stops thinking about. The instruction is simple enough — abdomen, thigh, or upper arm, and do not use the same spot twice in a row — but "twice in a row" is doing a lot of work in that sentence, and almost nobody tracks it.

Here is what actually happens over six months. The first few weeks, rotation is careful. Then a routine sets in: same time of day, same chair, same reachable patch of abdomen. Six months later there is a small area that has taken forty of the last fifty injections, and the tissue there has quietly changed.

Lipohypertrophy is the clinical term. The tissue thickens, and thickened tissue absorbs less predictably. That matters, because the whole point of a weekly GLP-1 is a steady, predictable exposure. Injecting into a hardened area can mean less drug absorbed than expected, and the experience of that is not dramatic — it is a slow sense that the medication is not doing what it used to, on a dose that has not changed.

The fix is boring and effective: write down the site every single time. Not a diagram, not a body map with coloured pins — just a word. "Left abdomen." "Right thigh." A four-week rotation across four named zones is enough structure for most people, and once the sites are in a list you can see at a glance whether the rotation actually happened or whether it drifted back to the same convenient spot.

Two other things worth noting alongside the site. First, any local reaction — redness, a lump, itching — and how long it lasted. Isolated reactions are common and unremarkable. A pattern of them in one zone is information. Second, whether the injection was into a fresh area or one used within the last month.

There is a related habit worth building: check the site before injecting, not after. Pinch the area and feel for firmness compared to the surrounding tissue. If it feels different, use a different zone and note it.

None of this needs an app. A note on your phone works. I use GLP-1 Dose Log because the site field sits next to the dose and date and takes one tap, and because a list of the last twelve injections makes rotation drift obvious in a way memory never does.

The general principle holds regardless of tool: the things you do not write down are the things you stop doing correctly.