Lipohypertrophy
A lump of thickened fatty tissue caused by repeated injection into the same spot. It matters because absorption through it is slower and less predictable.
Lipohypertrophy is a build-up of fatty tissue under the skin at a site injected repeatedly. It presents as a raised, often firm area, sometimes easier to feel than to see. It is the most common local complication of subcutaneous injection and it is well documented in insulin therapy, where it has been studied for decades.
The mechanism is a local growth response. Insulin and related peptides have anabolic effects on adipose tissue, and repeated delivery of the same compound into the same few square centimetres produces exactly what that describes. Contributing factors that recur across the literature are injecting into the same site, reusing needles, and injecting a large number of doses over a long period.
Why it matters more than it looks
The cosmetic aspect is the least of it. Absorption from lipohypertrophic tissue is slower and considerably more variable than from normal tissue. The tissue is poorly vascularised, so the compound leaves the depot at a different rate, and that rate is not consistent from one injection to the next.
The practical consequence is that dose stops meaning what it did. The same number of units injected into an affected site produces a different, less predictable exposure than into healthy tissue. In insulin therapy this shows as unexplained variability, and the same physical logic applies to any compound delivered subcutaneously.
There is a second-order trap. Because absorption from an affected site is reduced, the apparent response to a given dose falls, and the intuitive correction is to increase the dose. That increases the amount going into the tissue driving the problem. Recognising the site rather than adjusting the number is the part that gets missed.
What rotation actually means
Rotation is the standard preventive measure, and it is routinely done in a way that does not work. Moving between the abdomen and the thigh is rotating between regions. Within each region, injections still need to be spaced.
The structured version taught in diabetes care divides a region into quadrants, uses one quadrant at a time, moves to the next after a defined period, and spaces individual injections within a quadrant by roughly a finger's width. The principle is that no single point should receive repeated injections over a short span.
Absorption also differs between regions in the first place, which is a separate reason to be systematic. Switching regions changes the absorption profile as well as the site, so rotating within a region and changing regions are not equivalent moves.
Finding it
Affected tissue is frequently easier to detect by touch than by sight, particularly early. Clinical guidance for insulin users is to inspect and palpate sites regularly, and to stop using an affected area until it resolves. Recovery is generally described as taking months of avoidance, so this is a problem best not established.
If you are injecting regularly, keeping a record of where each dose went is the practical version of rotation. That is one of the things the injection site map in the app is for.
Not the same as lipoatrophy
The two get used interchangeably and they are opposites.
Lipohypertrophy is tissue gain: a raised, thickened lump where fat has accumulated. Lipoatrophy is tissue loss: a depression or dent where subcutaneous fat has been destroyed. Lipoatrophy is understood to be immune-mediated rather than a growth response, and it became rare after purified and recombinant insulins replaced the earlier animal-derived preparations.
Both are local reactions at injection sites and both distort absorption, but they have different causes and the same site cannot be described by both words. If a site is raised, that is hypertrophy. If it is sunken, that is atrophy, and it is worth a clinician looking at it, because the mechanism is different.
What this has to do with logging
Everything above is a pattern that only becomes visible over time. A single injection tells you nothing; forty injections clustered in one quadrant tell you a great deal. Absorption variability shows up the same way, as a trend rather than an event.
This is the ordinary argument for keeping a record of route and site alongside dose rather than dose alone. Two identical doses are not identical if they went into different tissue, and without a record there is no way to know which one you gave.
Related terms: subcutaneous, insulin syringe, intramuscular.
