Delayed emptying explains some of this, and not all of it

The apparent contradiction of a drug that causes both constipation and diarrhoea resolves once motility is separated from transit. Receptor activation slows antral contraction and gastric emptying and reduces small-bowel transit, which favours constipation. It also alters fluid handling and, in some people, produces bile-acid-related loose stool and post-prandial urgency. Different segments of a long tube, different effects, one drug. Roughly a third of participants in the large trials reported diarrhoea and roughly a quarter reported constipation, and a substantial number reported both at different times.

The STEP 1 gastrointestinal rows, in full

The pivotal semaglutide obesity trial randomised 1,961 adults to 2.4 mg weekly or placebo for sixty-eight weeks. Gastrointestinal disorders were reported by around seventy-four per cent of the active arm and about forty-eight per cent of placebo. Within that, nausea was reported by roughly forty-four per cent against seventeen per cent, diarrhoea by about thirty-two per cent against sixteen, vomiting by about twenty-five per cent against seven, and constipation by roughly twenty-three per cent against ten.¹

Three features of that table are routinely lost. The placebo rates are high, which is what happens when a large population is asked systematically about gut symptoms every few weeks. The events were predominantly graded mild or moderate. And discontinuation attributable to gastrointestinal events ran to about four and a half per cent of the active arm, against under one per cent on placebo.

The gap between three-quarters of participants reporting a gastrointestinal event and four and a half per cent stopping because of one is the most informative thing in the table. Most of this effect profile is endured rather than disabling, and any account that quotes the first figure without the second is describing something other than what happened.

Why the mechanism is central, not gastric

The area postrema lies in the floor of the fourth ventricle, has an incomplete blood-brain barrier, and therefore samples circulating substances directly. It expresses the GLP-1 receptor, it is the chemoreceptor trigger zone, and it projects into the nucleus tractus solitarius, which integrates peripheral satiety signalling. Appetite suppression and nausea are generated by overlapping circuitry in the same small region.²

That anatomy sets the ceiling of the class. It is not possible, with a molecule that acts at this receptor, to engage the satiety pathway strongly without engaging the emetic pathway to some degree, because the neurons are neighbours and in some cases the same neurons. Attempts to separate the two pharmacologically are among the more interesting things in the current pipeline, and part of the interest in dual agonism is precisely the possibility that a second receptor arm reduces the nausea penalty for a given degree of satiety.

The practical consequence for a reader is that antiemetic strategies aimed at the stomach address the minor route and leave the major one untouched. It is also why nausea in this class often has the quality patients describe as unrelated to food.

The gastric-emptying data, and its limits

Emptying delay in this class has been measured by scintigraphy, by paracetamol absorption and by stable-isotope breath test. The consistent findings are that delay is dose-dependent, largest in the early weeks at a given dose, and subject to partial tachyphylaxis over subsequent weeks of unchanged exposure.³

Three limits on that data matter. Most studies were small. Between-individual variability in measured emptying rate is large, which means population means conceal people at both extremes. And the relationship between measured emptying delay and reported symptoms is looser than intuition suggests: some people with substantial delay report little, and some reporting a great deal have unremarkable measurements.

The residual delay at steady state is the part relevant to procedures. It is smaller than the early delay and it does not disappear, which is the entire basis for perioperative concern. The Journal notes that the studies underlying that concern were not designed as perioperative risk assessments and that using them as such is an extrapolation — a reasonable one, and an extrapolation nonetheless.

One drug, two opposite bowel effects

Receptor activation slows antral contraction and gastric emptying and lengthens small-bowel transit, which favours harder, less frequent stool. Simultaneously, altered bile-acid delivery and changes in fluid handling produce loose stool and post-prandial urgency in a substantial minority. Different segments of a long organ respond differently, and a single participant can report both terms in the same trial at different times.

There is a second contributor that has nothing to do with receptors. Intake falls sharply on this treatment — that is the point — and stool volume falls with it. A person eating half of what they ate six months ago will pass less, less often, and will frequently interpret that as constipation when it is a change in throughput. Distinguishing reduced volume from genuine slow transit changes what should be done about it.

Fibre intake usually falls faster than total intake, because appetite suppression tends to displace bulky, low-energy-density foods first. That is an under-recognised route to constipation on this treatment and one of the few places where a dietary intervention has an obvious mechanistic target rather than a general plausibility.

Retained gastric content: what was actually found

The perioperative concern began with case reports and grew with retrospective series. A retrospective analysis of patients undergoing elective procedures found increased residual gastric content in those taking semaglutide despite standard fasting, and subsequent endoscopic and gastric-ultrasound studies have generally, though not universally, pointed the same way.⁴

The professional response moved in two stages. An initial position advised withholding the agonist before elective procedures — a week for weekly formulations. A subsequent multisociety statement, drawing on more data and on the observation that omitting a single weekly dose does not clear a drug with a week-long half-life, replaced the blanket approach with an individualised assessment considering symptoms, dose stability, procedure type and the option of extended clear-liquid fasting or point-of-care gastric ultrasound.⁵

The Journal regards this as a reasonable evolution and notes what it implies: the first guidance was issued on thin evidence because the alternative was silence, and it was revised when better evidence arrived. That is how this is supposed to work, and it is worth saying so in a field where guidance changes are usually reported as reversals.

Pancreatitis, with its actual numbers

Acute pancreatitis has followed this drug class since the earliest incretin products, driven initially by case reports and pharmacovigilance signals. The large randomised outcome programmes provide the best available evidence, because they adjudicated events and had comparator arms in populations with an elevated background rate. In the liraglutide cardiovascular outcome programme, adjudicated acute pancreatitis was rare and did not show the imbalance the earlier signal suggested.⁶

Two secondary findings from that work are useful. Asymptomatic elevations in amylase and lipase are common on treatment and are not diagnostic of pancreatitis, which means an incidental enzyme result should not by itself prompt discontinuation. And prior pancreatitis, while an exclusion in many trials, has not been shown to convert into a demonstrable recurrence signal on treatment.

The clinical marker remains what it has always been: severe, persistent epigastric pain, often radiating to the back, often with vomiting that does not settle. That presentation is not the expected effect profile of this class and should be treated as an urgent assessment rather than a titration question. The Journal reports the reassuring randomised data and declines to convert it into a statement that the event does not occur.

When the symptom is not the molecule

Everything above assumes the vial contains the compound at the stated strength and nothing else of consequence. For licensed product that is a fair assumption. For research-grade material it is a hypothesis, and it bears directly on symptom interpretation, because a person cannot reason about tolerability if the exposure is unknown.

Three failure modes produce gastrointestinal symptoms that will be misattributed. Peptide content below the labelled figure means a person is at a lower dose than they believe, and escalating on that basis produces a larger real step than intended. Content above the labelled figure does the reverse. And bacterial endotoxin, which is not detected by any purity assay, produces systemic symptoms including nausea, chills and malaise that look nothing like a specification failure on paper.

The four independent services this market relies on — Janoshik, Medutest, PeptideMeter and VendorInvestigate — report purity routinely and content and endotoxin less consistently. Several vendors, among them WXT, SSA, CPC and SWB, now publish per-batch reports; several do not. The Journal has argued in Analytics that content and endotoxin should be standard reported fields, and the tolerability case is the strongest argument for it we know.

The record that makes a symptom interpretable

Nearly every question a person asks about a gastrointestinal symptom on this treatment turns on information that is easy to record and hard to recall. What the current dose is. What date the current dose began. Whether the symptom is better, worse or the same than it was seven days ago. Whether fluids are being kept down. And whether anything else changed in the same week — a new vial, a new supplier, a new medication, an illness.

With that, a clinician can distinguish a first-week escalation effect from something else, can tell whether the trajectory is the expected improving one, and can attribute a change in tolerability to a change in material rather than to the drug. Without it, the consultation runs on recollection, and recollection about nausea is unusually poor.

We make no claim that a diary improves outcomes; that has not been tested and we would be sceptical of a trial claiming it. The narrower claim is that it converts an anecdote into a datum, and a substantial part of what this market believes about tolerability is currently anecdote reported at scale.

How the Journal reports adverse-event figures

Four conventions govern the numbers here. Incidence is quoted with the comparator arm alongside it, always, because a drug figure without a placebo figure is uninterpretable in a symptom domain with a high background rate. Figures are identified as cumulative participant incidence rather than prevalence. Where a figure comes from a pooled analysis or a post-hoc tolerability paper rather than a primary publication, we say so. And observational associations are labelled as such and never described in causal language.

Where we report practice rather than evidence — which in the management sections is most of it — the text states that the recommendation rests on mechanism or on transfer from another population. We would rather publish a short list of supported measures and a labelled longer list of reasonable ones than a single confident list that conceals the difference.

Nothing in this file is medical advice. The Journal does not diagnose, does not recommend medicines or doses, and cannot assess an individual. Several compounds discussed are sold for research use only and are not approved for human use in any jurisdiction. Symptoms that are severe, persistent or worsening warrant assessment by a clinician who can examine the person concerned.

A short glossary of terms that get swapped

Nausea: the sensation preceding or in place of vomiting; a symptom. Vomiting: forceful expulsion of gastric contents; a sign. Retching: the effort without the expulsion. Early satiety: fullness disproportionate to volume consumed. Dyspepsia: upper abdominal discomfort, often used loosely to include all of the above.

Gastroparesis: a clinical diagnosis of delayed gastric emptying with characteristic symptoms and no mechanical obstruction. It is not a synonym for drug-induced emptying delay, and the two are conflated constantly. Ileus: failure of propulsion without mechanical obstruction. Obstruction: mechanical blockage.

Incidence: proportion of a population experiencing at least one event in a period. Prevalence: proportion affected at a point in time. Adverse-event tables report the first and are read as the second. Adjudicated: reviewed against predefined criteria by a committee blinded to treatment, which is a materially stronger standard than a reported term.

The gastrointestinal effect profile of this class is well characterised at the population level and poorly characterised at the level of a single person on a single Tuesday. That asymmetry is the source of most of the frustration around it. The trials tell you accurately what proportion of a large group reported nausea; they cannot tell you whether the nausea you have at week nine will settle. What they do offer is a documented pattern — dose-related, escalation-clustered, attenuating at a fixed dose — against which an individual experience can at least be compared.