GLP-1 and Constipation: the Label Says 24%, and the Placebo Arm Says 11%

Almost every answer to "does Ozempic cause constipation?" quotes a single percentage from the drug's side. Almost none of them quotes the number sitting directly beside it in the same table: the share of people on placebo who reported the same thing. For constipation that omission matters more than for any other common GLP-1 side effect, because the placebo number is large.

This is the eighth piece in our evidence series, after which effects have enough evidence to calibrate a model, the corpus-derived pooled rates, what patients themselves report, gallstones end to end, what is inside a personalised risk number, hair loss end to end and why trial vomiting rates run from 6.5% to 58%. Like those, every figure below is quoted from a primary source with the arm and the number of participants it belongs to, and every one of them was read on 9 September 2026 — from the current US prescribing information, from each trial's own results table on ClinicalTrials.gov, or from our own live production API.

What the label says

The current US prescribing information for Wegovy (semaglutide), revised June 2026, lists constipation among the *"most common adverse reactions (incidence >= 5%)"*. Its three adverse-reaction tables each give a drug column and a placebo column:

Table 3 is the source of the 24% that gets quoted. Read the whole row and it says something different from the headline: 24 in a hundred on the drug, 11 in a hundred on placebo, in the same trials, over the same 68 weeks, under the same reporting rules. (Quoted from the prescribing information via DailyMed, SPL version 19, effective 18 June 2026, accessed 9 September 2026.)

For comparison, the same Table 3 puts nausea at 44% against 16% on placebo, diarrhoea at 30% against 16%, and vomiting at 24% against 6%. Constipation's placebo column is not the highest in the table — nausea and diarrhoea both sit at 16% — but constipation is the effect where the placebo share is the largest fraction of the headline: 11 of 24, against 6 of 24 for vomiting.

What the trial registries say, arm by arm

Our database cites 13 distinct studies for a stated constipation rate. Nine of them post a placebo arm in their public results, and eight of those nine have placebo arms large enough to divide. Every row below was read from that trial's own ClinicalTrials.gov adverse-events module on 9 September 2026; the counts are the registry's, the percentages and the differences are our division and subtraction of them.

The other four sources in our constipation base are absent from this table for two different reasons. Three post no placebo arm at all: tirzepatide versus dulaglutide (NCT04255433), where both arms are active GLP-1 drugs and the constipation rates are 836/6,647 = 12.6% and 771/6,647 = 11.6%, and two all-active orforglipron studies (NCT06010004, NCT06440980). The fourth, a placebo-controlled semaglutide trial in non-alcoholic steatohepatitis (PubMed 33185364), does have a placebo group, but our corpus holds it as a published abstract rather than a registry results table, so we have no per-arm counts on file for it and are not going to quote a difference we have not read. And the ninth placebo-controlled study, NCT05086445, posts placebo arms holding four and eleven participants, which is too few to divide.

The difference, not the headline

Take the difference column seriously and the picture is tighter than the raw rates suggest — inside one setting, and only inside it.

In the weight-reduction trials the label itself pools, and in their own registry records, the gap is 11 to 14 points in every source that reports it: Table 3 (24 − 11 = 13), Table 4's 2.4 mg column (19 − 8 = 11), STEP 1 (23.4 − 9.5 = 13.9) and STEP 2's 2.4 mg arm (17.4 − 5.5 = 11.9). The raw drug figures across those same four sources run from 17.4% to 24% and the placebo figures from 5.5% to 11%, so the two columns move together. Those four readings are not four independent trials, and we are not going to present them as if they were: Table 3 is the label's pool of three weight-reduction trials, and STEP 1 and STEP 2 are trials inside that programme with their own registry records, so the overlap is deliberate — the point is that the difference survives being computed at both the pooled and the single-trial level, not that four separate studies agree. Oral semaglutide at 50 mg lands in the same band (+11.8), and tirzepatide's two lower doses in SURMOUNT-1 sit just inside it (+12.0 and +12.5).

Outside that setting the difference is not stable, and we are not going to present it as if it were. SELECT, dosed identically at 2.4 mg, records 8.1% against 2.6% — a 5.5-point gap with both arms about a third of the STEP 1 values. The most likely reason is not that the drug behaves differently but that a cardiovascular-outcomes trial in 17,604 people ascertains and reports a mild gastrointestinal event differently from a 68-week weight-management trial that is looking for it; that is a hypothesis about trial conduct, not something either record states. At the other end, the three orforglipron arms in the Japanese dose-ranging study sit 20 to 28 points above their own placebo, and the mechanistic appetite study 36 points above its own — small trials in specific populations on an oral small molecule, not the injectables the label describes. And tirzepatide 15 mg (+6.8) reports *less* constipation than tirzepatide 10 mg (+12.5) in the same trial, which is a reminder that these are counts of reported events, not a dose-response curve.

So the honest one-line answer to "how much constipation does the drug itself add?" is: in adult weight-reduction trials of injectable semaglutide 2.4 mg, about 11 to 14 people in every hundred, on top of a background of 5 to 11 in every hundred who report it on placebo. Any other setting needs its own number.

Why our own pooled figure is 11.8%, and why you should not quote it as the Wegovy rate

Our constipation endpoint publishes a pooled clinical estimate of 11.8% (95% CI 1–56%), from 25 eligible stated rates across 13 distinct studies, source diversity "high". That is roughly half the label's 24%, and the reason is entirely in what the pool contains.

Our pooled figure weights each source by its own stated sample size (a source stating none counts as 1), multiplied by a discount for how confident our extraction was in reading the figure — so the two largest studies dominate it: SELECT (8.1%, n=8,803) and the tirzepatide-versus-dulaglutide trial (12.1% across its two active arms, n=6,647 each). Both are large, both are real, and neither is a semaglutide 2.4 mg weight-reduction trial. Mixed in beside them are three orforglipron studies of an oral small molecule, a retatrutide dose-ranging study, an oral semaglutide tablet trial and a mechanistic study in 72 people. Pooling all of that answers the question "across everything we can cite about GLP-1-class drugs, what does the constipation literature look like?" It does not answer "what happens on Wegovy", and the 95% interval spanning 1% to 56% is the honest statement of how much those sources disagree.

If you want the Wegovy 2.4 mg number, quote the label: 24% against 11% on placebo, N=2,116 and N=1,261, Table 3. If you want the whole class with its heterogeneity visible, quote ours, with its interval and its source count. Quoting our pooled percentage as though it were the label's number would be wrong in the direction of understating, which is the kind of error that survives longest because nobody complains about it.

Nobody's source says what causes it

It is close to universal in GLP-1 content — and it was on our own site until this week — to explain constipation by saying the drug slows gastric emptying. The label does say semaglutide delays gastric emptying: it is a named subsection of the Pharmacodynamics section, *"Gastric Emptying: Semaglutide delays gastric emptying"* (section 12.2), and the Drug Interactions section warns that this may affect the absorption of oral medicines taken alongside it (section 7.2). What the label never does is connect that statement to constipation. We searched every occurrence of the word in the current SPL: it appears in the highlights list of common reactions, in each of the three adverse-reaction tables, in the paediatric narrative and in the postmarketing list — and not once with a mechanism attached.

We had published a sentence attributing this effect to slowed stomach emptying in the description our API and predictor serve for it. It was a plausible sentence that no source we hold states, and it was withdrawn on 9 September 2026; the field now quotes the label's figures and stops there. We are describing the correction rather than quietly editing it because a confident mechanism claim with no citation is exactly the sort of sentence this series exists to remove — and it is worth knowing that the version you may have read on a dozen other sites has, as far as the prescribing information is concerned, no stated source either.

How this number moved twice in three days

Anyone citing a live figure deserves to know when it moved and why. Constipation's published estimate changed twice in the same week, both times because of defects in our own pipeline, both documented in our methodology's corrections log:

7 September 2026. A newly pinned fetch of pivotal trials' results started emitting the registry's *serious*-adverse-event table alongside its non-serious one, and our collector had been concatenating the two since August. Serious constipation counts are tiny by construction — STEP 1 posts 1 of 1,306 in the serious table against 305 of 1,306 in the non-serious one — so pooling them as incidence dragged the published constipation estimate from roughly 28% to roughly 5% in a single sync. Those 39 rows were withheld the same morning and the collector now reads the non-serious table only.

9 September 2026. The withheld serious rows stayed in the store, and our duplicate check asked only whether a row for that trial and effect existed, not whether it carried any evidence. STEP 1's suppressed serious row was therefore telling the collector that STEP 1's real constipation row — the 305 of 1,306 above — had already been collected. Recovering it moved the published estimate from 10.9% to 11.8% and the base from 24 rates across 12 studies to 25 across 13.

Neither correction was found by an outside reader, and both are the reason the figures in this article are stated with the date they were read. The corpus updates daily; the live endpoint is always the current answer.

What patients themselves report

Of the 26 distinct community reports in our corpus, 7 mention constipation — a 26.9% reporting frequency. That is not an incidence rate and we never treat it as one: it is the share of people who chose to post about their GLP-1 experience who mentioned constipation at all. See that article for why the pool is small and cannot currently grow.

If you are on a GLP-1 now

This is educational content, not medical advice. Two things in the label are worth knowing rather than paraphrasing. Constipation is listed among the most common adverse reactions in the weight-reduction trials, at rates well above placebo. Separately, the postmarketing section — reports collected voluntarily after approval, where the label itself says it is "not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure" — lists *"ileus, intestinal obstruction, severe constipation including fecal impaction"* among reported gastrointestinal reactions. Constipation that does not resolve, or that comes with abdominal pain, vomiting or an inability to pass gas, is a reason to contact a clinician rather than to add more fibre. Nothing here should be used to start, stop or change a medication or its dose without your prescriber.

Every figure in this article was read on 9 September 2026 from the cited primary source or from our live production API, and is stated as of that date. Query everything behind it — all 15 effects, both tracks, every source with its URL — at magistra.health/en/data-api, with the full eligibility methodology at magistra.health/en/methodology.