Injection Site Reaction: What Counts and How Often
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[Glossary · Injecting]

Injection site reaction

Also called ISR, injection site redness, injection site bruising · Reviewed

Definition: An injection site reaction is any change at the spot where a shot went in: redness, swelling, itching, pain, bruising or bleeding. Labels report how often these happened in trials, and the rate differs a lot from product to product.

How often labels report them

Injection site reactions in trials, per the current US label.
ProductTreated groupPlacebo group
Wegovy (semaglutide), adult weight trials1.4%1%
Mounjaro (tirzepatide), placebo-controlled trials3.2%0.4%
Xyosted (testosterone enanthate), up to 1 year12.7%No placebo group

The Xyosted label lists bruising, bleeding and redness among the reactions, and says no patient stopped the drug because of them. These figures belong to each tested product. They do not transfer to compounded or research versions.

Worked example

Reading a label percentage

  1. Mounjaro: 3.2% means about 3 in 100 treated people reported a reaction.
  2. Placebo: 0.4% means about 4 in 1,000 on a dummy shot did too.
  3. So most reactions were linked to the drug, but the shot itself causes some on its own.

Why it matters when you track doses

A short note with the dose ("red patch, gone next day") turns a vague memory into a pattern you can show a prescriber. If reactions cluster on one site, one vial or one batch, the log will show it. Talk to your prescriber about any reaction that grows, spreads or does not fade.

A reaction is different from lipohypertrophy, a lump that builds up slowly from repeated shots in one place.

Read more on Dose Track: Mounjaro guide · Wegovy guide

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
Injection site rotationInjection site rotation means moving each injection to a different spot instead of using the same patch of… LipohypertrophyLipohypertrophy is a rubbery lump of thickened fat under the skin that forms where injections are given… Subcutaneous injection (SubQ)A subcutaneous injection (SubQ) puts medicine into the layer of fat just under the skin, above the muscle.… EndotoxinEndotoxin is a substance from the outer wall of certain bacteria (gram-negative bacteria), released when…

Frequently asked questions

How common are injection site reactions with GLP-1s?+
The Wegovy label reports 1.4% vs 1% on placebo in adult weight trials. The Mounjaro label reports 3.2% vs 0.4% on placebo.
Do people stop treatment because of them?+
The Xyosted label says no patient stopped because of injection site reactions. Any reaction that grows, spreads or does not fade is worth raising with your prescriber.
Is a lump the same as a reaction?+
Not usually. A reaction comes and goes after a shot. A lump that builds up over weeks at a reused spot is more likely lipohypertrophy.

Sources

  1. Wegovy (semaglutide) US prescribing information, DailyMed
  2. Mounjaro (tirzepatide) US prescribing information, DailyMed
  3. Xyosted (testosterone enanthate) US prescribing information, DailyMed

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