Injection-Site Reaction (ISR)

An injection-site reaction is a local response at the point of subcutaneous or intramuscular administration, ranging from transient stinging and erythema through induration and pruritus to persistent nodules. Several mechanisms produce a similar appearance. Formulation contributes through pH far from physiological, non-isotonic osmolality, high excipient load or a vehicle designed to precipitate at the site. Immune mechanisms add a delayed local response. Contamination or endotoxin produces inflammation indistinguishable at the outset from any of these.

Rates depend more on formulation than on molecule. Depot and microsphere products that deliberately precipitate a polymer to release drug over weeks produce nodules far more often than isotonic aqueous solutions of the same active agent. Preservatives such as metacresol and benzyl alcohol, which multi-dose vials require, are a recognised source of stinging. Repeated injection into one small area produces lipohypertrophy, documented most thoroughly with insulin, where the altered tissue absorbs unpredictably enough to matter.

Most are a tolerability issue rather than a safety one, but a few features change the assessment. Spreading erythema with fever, fluctuance, or a lesion appearing several days after the injection rather than within hours points toward infection or a sterile abscess. Progressive nodularity is a pharmacokinetic problem as much as a cosmetic one, which is why site rotation is a dosing intervention rather than a comfort measure.

Two errors recur. One is escalating a local reaction to anaphylaxis, which needlessly abandons a workable therapy; the two differ in that anaphylaxis involves organ systems distant from the injection. The other is dismissing a local reaction as expected when it is the first sign of contaminated material.