GLP-1 Injection-Site Reactions: Two Trials State a Rate — One of Them From Seven People

Injection-site reactions are the side effect every GLP-1 injector expects — redness, a small bump, itching where the needle went in — and the one our own database has the thinnest formal evidence for. Of the 15 side effects we track, it sits near the bottom on distinct clinical sources: two trials, four stated rates between them, and a pooled estimate whose confidence interval runs from 0% to 53%. This piece lays out exactly what those two trials say, why the interval is that wide despite "low" being the worst it gets on our source-diversity label, and what the spontaneous-report data (FAERS) says instead.

The two trials, in full

Our clinical evidence base for this effect is 4 stated rates from 2 distinct sources, collected from ClinicalTrials.gov's own posted results:

(Read from the ClinicalTrials.gov API v2 adverse-events module on 21 September 2026.)

Two things worth noticing before the pooled number. First, SURMOUNT-1's own three arms are not dose-ordered — 10 mg reports a higher rate (5.7%) than 15 mg (4.6%), which is the kind of non-monotonic pattern you expect from a specific-term adverse-event count at this size, not evidence that a higher dose causes fewer reactions. Second, no placebo-arm rate is stored for this trial, for any effect: our collector deliberately skips placebo arms at collection time (fixed 6 September 2026, after two placebo-arm rates were found stored elsewhere in the corpus as if they were drug rates), so this effect's page cannot show a drug-versus-placebo comparison from our own data — that comparison, if you want it, has to come from the registry's own table directly.

The fourth row is the one doing the least pooled work and the most to widen the interval: a 7-person arm of a Phase 2 chronic-kidney-disease trial, where a single participant's erythema is 14.3%. One event either way would move that rate by more than 14 points.

Why "low" confidence still means a 0–53% interval

Our pooled clinical estimate reads 4.5% (95% CI 0–53%), source diversity "low" (the label applies at ≤3 distinct sources; we have 2). The weighting behind that number is lopsided on purpose: tirzepatide's 630–636-participant arms carry 98.9% of the pooled weight, the 7-person dulaglutide arm just 1.1% — sample-size weighting is doing what it should, discounting the smallest, noisiest source almost to nothing.

But `intervalBasis` on the same endpoint reads `sampling_plus_between_study`, not `sampling_only_single_source` — because two distinct sources exist, the between-study heterogeneity term (τ²) is no longer zero by construction, and with only two sources whose point estimates sit 9.9 percentage points apart (14.3% vs 4.4%), that heterogeneity term is large relative to the pooled mean. The result is a headline rate (4.5%) that is almost entirely tirzepatide's own number, sitting inside an interval wide enough to admit almost anything. Our own API's confidence-interval note warns about exactly this trap, the other way round from how it usually comes up: "a NARROW interval is not evidence of a well-known rate." This effect is the mirror case — a "low" source-diversity label sitting beside a WIDE interval — and it is the honest state here, not a defect in the label. (Our companion piece makes the same point from the opposite direction: our best-evidenced effect, nausea, publishes a wider range than several of our thinnest.)

What FAERS shows instead — a different kind of signal

FDA FAERS spontaneous reports tell a different story about salience, not incidence. Reading each drug's top-30 most-reported reaction terms (snapshot dated, see caveat below):

Neither semaglutide nor liraglutide has an injection-site term anywhere in its own top-30 most-reported-reaction list in our corpus — a factual absence in this ranking, not a claim that semaglutide or liraglutide users never report injection-site reactions.

Read this table for what it is, not for an incidence rate. openFDA's `shareOfReports` here counts term-mentions among a drug's 30 most-reported reaction terms, not patients or even reports — a single report naming three reactions is counted under all three, and every term outside that drug's own top 30 is in neither numerator nor denominator. Each row is also a one-time snapshot, never re-queried against the same URL, per the standing FAERS caveat on every effect page — and every one of the nine rows above is from April 2026: the three tirzepatide rows were scraped on 7 April 2026 and the six dulaglutide, exenatide and lixisenatide rows on 12 April 2026. (That caveat quotes a corpus-wide range of 2026-04-07 to 2026-08-13, which is the span across all 61 FAERS rows behind the 15 effects we publish; none of this effect's nine falls anywhere near the recent end of it.) Take the table as a five-month-old ranking of what gets reported most, which is a different question from what a trial arm measures.

The community track: real posts, but a rate we deliberately don't compute

24 points sit in what our API calls the community track for this effect — 21 patient-report rows plus 3 news-typed rows, which our effect endpoint groups into the same track, so read "24" as the track's size and not as 24 patient posts. Four of the 24 state a precise-looking percentage — "3% at low dose," "4% at medium dose," "5% at high dose," "12% user-reported" — attributed to sourceNames like "Diabetes forum injection site discussions" and "Aggregated user reports (Reddit, forums, reviews)." All four are excluded from the published rate, correctly: they carry no reproducible single post or study behind the number, the exact synthetic-aggregate pattern our rate-base eligibility rules exist to catch. A forum thread does not have an incidence rate; averaging what several unverifiable summaries claim is not measurement.

What's left is reporting frequency, a different and honest metric: of 26 distinct community reports corpus-wide, 6 (23.1%) mention injection-site reactions — 4 from our tracked GLP-1 subreddits (r/Semaglutide, r/compoundedtirzepatide, r/Zepbound twice) and 2 from Drugs.com patient reviews. Several more injection-related Reddit posts sit in the raw corpus but come from communities our collector isn't configured to read — r/TirzepatideRX, r/CompoundedSemaglutide, r/liraglutide and others are real drug-specific communities the tracked-subreddit list currently omits, the same gap an open founder decision (`community-screen-excludes-glp1-subreddits-2026-09-21`) is already weighing for every effect, not just this one. 23.1% is a share of mentions, not an incidence percentage, and it is not comparable to the 4.5% clinical figure above.

What it would take to narrow this

The interval on this effect is wide for a structural reason, not a collection failure: only two trials in our corpus post a comparable per-arm number for this specific term, and one of them has seven participants. A third trial posting this term — at any size — would move `intervalBasis` no further than it already sits (it's already past single-source), but a third, larger source would pull the between-study weight away from the 7-person outlier and should narrow the range. Until then, the honest summary is what the two primary sources actually say: two tirzepatide-dose arms and one dulaglutide arm cluster around 3–6%, a single small dulaglutide kidney-trial arm reports 14.3%, and the pooled estimate correctly tells you it cannot rule out either the low single digits or something much higher.

Reproduce it

Every number above was read from ClinicalTrials.gov's own posted adverse-event tables and openFDA's FAERS endpoint on 21 September 2026, and cross-checked against our local production build of `GET /api/data?q=effect&id=injection_site_reaction` the same day — the pooled estimate, its weighting, the exclusion counts and the reporting-frequency figure are all quoted from that endpoint's live output, not derived by hand. Query it yourself, or any of the other 14 effects, at magistra.health/en/data-api, with the full eligibility methodology at magistra.health/en/methodology.