Holding the dose before surgery: for whom, for how long

The kinetics create a difficulty that the first round of guidance did not fully address. Advising a patient to omit one weekly dose before a procedure reduces exposure by perhaps a quarter to a half depending on timing, which is not clearance in any meaningful sense. If the concern is a pharmacodynamic effect on gastric emptying, then omitting a single dose of a drug with a week-long half-life is a gesture rather than a solution, and the multisociety guidance issued subsequently reflects that arithmetic more honestly than the original advice did.

Incidence by dose in the tirzepatide programme

In the seventy-two-week tirzepatide obesity trial, nausea was reported by approximately twenty-five per cent at 5 mg, thirty-three per cent at 10 mg and thirty-one per cent at 15 mg, against about ten per cent on placebo. Diarrhoea ran between nineteen and twenty-three per cent across the dose range against about nine per cent, vomiting between eight and twelve per cent against under two, and constipation between seventeen and eighteen per cent against about six.¹

The dose-relationship is present but not monotonic in every term, which is characteristic of adverse-event data at this sample size and a useful reminder that these figures carry confidence intervals nobody prints. Discontinuation for adverse events ran between four and seven per cent across doses against under three per cent on placebo.

Comparing across programmes is a trap. The semaglutide and tirzepatide obesity trials differed in duration, population, escalation schedule and adverse-event collection detail, and the apparent difference in nausea incidence between them is not a clean molecular comparison. The only defensible head-to-head tolerability comparisons in this class come from trials that randomised both molecules, and there are few of them.²

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.³

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.⁴

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.

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.⁵

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.⁶

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.

The arithmetic problem with holding a dose

Advice to omit one weekly injection before a procedure runs into a kinetic difficulty. For a drug at steady state with a seven-day half-life, skipping a single dose leaves roughly half of accumulated exposure at the point that dose would have been due. Skipping two leaves about a quarter. Meaningful clearance requires three to four weeks off, which for many patients means a month of lost treatment for a day procedure.

That is why the revised guidance emphasises assessment over blanket withholding. If the relevant question is whether this particular stomach is empty on this particular morning, then it can be asked directly by ultrasound, and the answer is more informative than an inference from a dosing calendar.

The practical obligation on the patient side is disclosure. An anaesthetist told about the drug can extend clear-liquid fasting, image the stomach, modify induction technique, or defer. An anaesthetist not told can do none of those things. The Journal has heard from readers who did not disclose because the compound was obtained outside conventional supply and they expected disapproval. That is a comprehensible fear and an unacceptable trade, and clinicians reading this should understand their part in creating it.

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.⁷

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.

On the perioperative question we intend to keep reporting rather than editorialising, with one exception. The evidence is unsettled and the disclosure obligation is not. Anyone taking one of these compounds who is scheduled for sedation or anaesthesia should say so, including — especially including — where the compound came from outside conventional supply. Clinicians who make that disclosure feel costly are part of the risk.